The gaming of Mexico’s PR disparity cap

One distinctive feature of Mexico’s 500-seat Chamber of Deputies’ mixed-member majoritarian system – under which 300 members are chosen by plurality voting in as many congressional districts, while the remaining 200 seats are filled by party-list proportional representation (initially determined on a nationwide basis) – is a provision which caps party representation at a maximum of eight percentage points above its national proportion of votes cast for parties entitled to participate in the distribution of PR mandates (in addition, no party may have a total number of Chamber seats in excess of 300 mandates). However, in recent years some parties running in electoral coalitions have successfully gamed the system to circumvent the limits imposed by this provision, effectively diluting its intended compensatory effects.

The eight percent disparity cap is implemented by determining the maximum number of Chamber seats each party may obtain; if a party exceeds this amount, its PR seat allocation is reduced so that its total number of plurality and list seats is equal to the capped seat total. For example, in the 1997 legislative election – generally regarded as Mexico’s first truly free and fair vote – the then-ruling Institutional Revolutionary Party (PRI) won 38% of the total vote and 39.98% of the “effective national vote” (“votación nacional efectiva” in Spanish), that is the total number of votes cast for parties entitled to take part in the allocation of PR seats. Therefore, PRI could obtain at most 47.98% (39.98% plus 8%) of the Chamber’s 500 seats, or 239 seats, disregarding fractions. Since PRI had won 165 of 300 plurality mandates in the election, and had been initially assigned 80 PR seats, for a total of 245 seats – six above the capped total – the party’s list mandates were reduced to 74 seats (239 minus 165); the remaining 126 PR mandates were then distributed among the other four qualifying parties.

Prior to 2008, Mexican voters could choose either a party running alone or a coalition, but not a specific party running within a coalition. Therefore, coalitions were treated as parties when it came to allocating Chamber seats and determining the eight percent cap. However, a 2008 electoral reform allowed voters to choose parties running as part of a coalition, and at the same time limited coalitions to plurality elections only. Since then, the distribution of Chamber PR seats has been carried out entirely on an individual party basis, even though coalitions have often continued to determine the outcome of plurality races; most importantly, the allocation of plurality seats among coalition partners has been determined on the basis of officially registered coalition agreements (“convenios de coalición” in Spanish). In due course, larger parties running in coalition with smaller allies discovered they could game the cited provision by assigning plurality seats in the coalition agreement to their junior partners, while these in turn nominated candidates who were actually members of the larger party. In this manner, the larger party artificially reduced its total number of seats, thus making it less likely that it would hit the eight percent cap during the allocation of PR list mandates.

Before 2018, PRI and its ally the Ecologist Green Party of Mexico (PVEM) gamed the system in the manner described, but the scope of their scheme was comparatively limited, not least because PVEM didn’t have a particularly large number of Chamber plurality seats to begin with. Moreover, in 2015 Mexico’s Federal Judiciary Electoral Tribunal (TEPJF) ruled that parties could nominate candidates from a different political party, provided the parties had a coalition agreement. On the other hand, the stratagem pursued in 2018 by the “Juntos Haremos Historia” (JHH; “Together We’ll Make History”) coalition of the National Regeneration Movement (MORENA), the Workers’ Party (PT) and the Social Encounter Party (PES) was anything but limited. For the Chamber of Deputies, the three parties ran together in 292 of Mexico’s 300 congressional districts, but in the coalition agreement 142 nominations were assigned to MORENA, and 75 each to PT and PES.

To be certain, it is not unusual for smaller parties like PT and PES to have a disproportionate share of single-member nominations in mixed-member majoritarian systems which allow party coalitions: in a nutshell, larger parties often bow to smaller parties’ demands, outrageous as they may be, if that is what it takes to avoid the risk of losing elections. That said, giving away half the Chamber’s single-member seat nominations to a couple of very small parties came across as both extreme and highly suspicious, all the more so considering that for the Mexican Senate plurality races, JHH nominated 49 candidates from MORENA, but only eight from PES and five from PT. However, unlike in the Chamber of Deputies, there is no eight percent cap in place for the distribution of Senate PR seats, and therefore MORENA gained nothing from giving away upper house plurality nominations to its much smaller allies.

JHH went on to score a landslide victory in the general election last July, securing an absolute majority for its presidential candidate Andrés Manuel López Obrador (better known by his initials AMLO), as well as a sizable plurality in the legislative election. In the Chamber of Deputies, final results show JHH won 220 of 300 plurality seats (including eight mandates won by MORENA running alone), with the following results for its constituent parties:

Party	% (Votes)	Seats	% (Seats)
MORENA	37.2	106	35.3
PT	3.9	58	19.3
PES	2.4	56	18.7
JHH Total	43.5	220	73.3

The sole beneficiaries of the victorious coalition’s over-representation in the plurality component were PT and PES, as MORENA ended up slightly under-represented. Again, while it is not unusual for smaller parties within larger coalitions in mixed-member majoritarian systems to have a share of seats in excess of their percentage of the vote (even without considering the PR component), this was nonetheless a truly extreme outcome. By comparison, the Senate election had a more typical plurality outcome, as shown below:

Party	% (Votes)	Seats	% (Seats)
MORENA	37.4	42	43.8
PT	3.8	5	5.2
PES	2.3	8	8.3
JHH Total	43.6	55	57.3

(JHH’s smaller majority of upper house plurality seats was due to the fact that unlike in the Chamber of Deputies, Senate plurality mandates are allocated on the basis of two seats for the winning ticket and one for the first runner-up in each one of Mexico’s thirty-two federal entities).

For the distribution of Chamber PR seats, MORENA received 41.34% of the effective vote, so it could have no more than 49.34% of the 500 Chamber seats, which translated to 246 seats. However, after the initial allocation of PR mandates MORENA had a total of 188 seats (106 plurality and 82 list), well below its corresponding maximum seat total. On the other hand, PT, with 4.36% of the effective vote, would have been entitled to nine list seats, but unexpectedly exceeded its 12.36% cap, or 61 seats due to the party’s large number of plurality victories; PES fell below the three percent valid vote minimum required to take part in the distribution of PR seats. Consequently, PT’s list seat total was reduced to three mandates, and the remaining 197 PR seats were distributed among the other six qualifying parties. In all, JHH won 308 of 500 Chamber seats, as detailed below:

Party	FPTP	PR	Total	% (Total)
MORENA	106	85	191	38.2
PT	58	3	61	12.2
PES	56	0	56	11.2
JHH Total	220	88	308	61.6

Mexico’s National Electoral Institute’s General Council Agreement INE/CG1181/2018 [PDF] details the official distribution of Chamber of Deputies PR list mandates in Mexico’s 2018 federal election, while Federal Elections in Mexico has nationwide- and federal entity-level results of elections in Mexico since 1997.

Although the eight percent cap deprived PT of just six seats, this left several of its leaders without a seat in the Chamber of Deputies, and the party went to court to have the result overturned. To that end, PT actually admitted some of its successful plurality candidates weren’t really party members after all, and sought a judicial review of JHH’s officially registered coalition agreement. However, the court rejected what it correctly perceived as a transparent attempt by PT to re-structure the previously agreed distribution of Chamber plurality nominations, in order to bypass the eight percent cap and increase its number of list mandates. In essence, TEPJF ruled JHH’s coalition agreement was a done deal which could no longer be altered after the election had been carried out with the agreement in place, and for good measure reaffirmed its earlier 2015 ruling that parties could nominate candidates from a different political party, provided the parties had a coalition agreement.

Just as important, TEPJF ruled that under the prevailing legal and constitutional arrangements, it was not possible to consider JHH’s coalition performance as if it were a single party for the purposes of determining over-representation, as it would run counter to the requirement such determination must be carried out on an individual party basis. Had Chamber seats been allocated among coalitions (and had all three major coalitions been in place in every congressional district, as was the case with the presidential election), JHH’s seat total would have been limited to 269 mandates – a significantly reduced yet nonetheless substantial majority.

TEPJF also confirmed that PT exceeded the eight percent cap by six seats (because 12.36% of 500 seats equals 61.8 seats, it was argued the party went above the cap by 5.2 seats (67 minus 61.8), which should have rounded down to five seats; however, this argument was rejected on the grounds that with 62 seats of 500 – 12.4% of the total – PT would have had a disparity gap of 8.04%, a figure just above the eight percent maximum).  The court ruled as well that JHH’s plurality victories could not be assigned to MORENA on account of the fact that the party contributed the vast majority of votes to that end.

(MORENA outpolled both PT and PES in every congressional district, and even if the party had run alone it would have secured a large majority of plurality seats, while PT and PES would have won none. However, in such hypothetical scenarios – which assume an identical distribution of votes among the parties contesting the 2018 federal election – MORENA’s Chamber of Deputies seat total would have been limited to 246 mandates by the eight percent cap, while PT would have received no more than 13 list mandates and PES would have had no seats, leaving MORENA and PT with a relatively slender joint majority in the Chamber.)

Nevertheless, TEPJF’s rulings on some of these controversies were not unanimous. Judge Felipe de la Mata Pizaña wrote a concurring opinion calling for the current implementation of the eight percent cap provision to be reconsidered for future elections, while Judge Reyes Rodríguez Mondragón issued an extensive dissenting opinion, in which he argued that the actual party affiliation of plurality winners should be taken into account for determining the eight percent cap during the distribution of PR seats. Notably, the dissent cites a number of political science scholarly sources, including both the Spanish-language edition of “Presidents and the Party System” (edited by S. Mainwaring and M.S. Shugart) and “Seats and Votes” by R. Taagepera and M.S. Shugart, even carrying out the calculations to determine the effective number of parties index for Mexico.

(For further information, please refer to TEPJF’s rulings on SUP-JDC-0429-2018; SUP-JDC-0438-2018; SUP-JDC-0444-2018; SUP-REC-0934-2018; and SUP-REC-0943-2018 [PDF].)

However, beyond the practical difficulties entailed by implementing this approach, it is not clear that it would have resulted in a significantly different distribution of seats. After Mexico’s newly elected legislators were sworn in just over three months ago, a large number of JHH’s 220 plurality deputies ostensibly elected as members of PT or PES joined MORENA’s parliamentary group, as shown below:

Party	Elected	Group	Gain/Loss
MORENA	106	162	+56
PT	58	28	-30
PES	56	30	-26

Had the eight percent cap been calculated on the basis of parliamentary group affiliation figures, the initial distribution of PR list seats, which as previously noted granted nine mandates to PT and 82 to MORENA would have been definitive, as neither party would have reached its eight percent-capped seat total. Moreover – and quite ironically – JHH would have obtained an even larger Chamber majority of 311 seats (of which 244 seats would have gone to MORENA, 37 to PT and 30 to PES).

In conclusion, while PT and PES contributed very little in terms of votes to JHH’s sweeping victory in the 2018 federal election, the gaming of the eight percent disparity cap through the assignment of a disproportionately large number of plurality nominations to these parties allowed the coalition to obtain a much larger majority in the Chamber of Deputies than it could have otherwise attained. Even so, the triumphant coalition parties overplayed their hand, and the stratagem backfired on PT; this in turn led to a public disclosure of the scheme’s existence (already highly suspected, not least because it was largely absent in the Senate election, which has no disparity cap provisions), as part of an unsuccessful effort to have the unexpected adverse outcome reversed in court.

Even though a majority of Mexico’s Federal Judiciary Electoral Tribunal justices have reaffirmed the validity of electoral provisions which allow outcomes like these, it seems an obvious flaw in the design of the system that parties can run in alliances, but the caps on over-representation are calculated at the individual party level, as pointed out by Dr. Matthew Shugart himself on this blog shortly after the 2018 federal election took place. Moreover, the gaming of the eight percent disparity cap in the 2018 federal election effectively altered the nature of the Chamber of Deputies electoral system, causing it to function almost like a parallel system, in which the outcome of plurality races had very little impact in the distribution of party list seats. That may not appear to be cause for excessive concern in the here and now, given that JHH would have in all likelihood secured a majority of Chamber seats even if the coalition hadn’t gamed the eight percent disparity cap, but the issue could have wider ramifications with unpredictable consequences in the years to come.