Mexican elections 2015

In addition to Turkey, about which we already have an active thread, there also are elections today in Mexico.

The entire Chamber of Deputies is being elected today, along with governors in nine states. Those nine states, plus some others, have elections for their state assemblies.

This is the first midterm election under a president from the Institutional Revolutionary Party since 1997. Each of the preceding has seen the familiar “midterm decline” phenomenon, whereby the president’s party sees its seat share of the legislature reduced in an election near the mid-point of the president’s term. Will this one defy the trend?

An average of polls that I have seen* put the PRI on around a third of the vote, which is approximately what it won in 2012. The National Action Party (PAN) also was polling around the same range as it won in the last election (around 27%). The difference maker could be the left, where the Party of the Democratic Revolution suffered a split: former leader Andres Manuel Lopez Obrador set up a new party, Morena, after the 2012 election. This party will compete with the PRD (and allies) for votes from the left.

The Mexican electoral system is a mixed-member system. Its PR-list component is neither fully compensatory nor fully parallel, but it is somewhat more the latter. That is, a party that wins a lot of the 300 single-seat districts will tend to be over-represented, even after the 200 list seats are allocated.** For instance, in 2012, the alliances of the PRI (33.6% of the vote) and Greens (PVEM, with 6.4%) took 241 seats (48.2%).

Given the disproportionality of the electoral system, it is not out of the question that the PRI (and PVEM ally) could gain seats even if it loses some votes from 2012. In much of the center and south of the country, the PRI’s main competitor is the PRD. With the PRD split, the PRI could pick up districts (which are decided by plurality) that the PRD won last time, even if its votes did not increase.

The caveat is that I have not looked at district-level patterns from 2012, nor have I seen state-level (let alone district-level) polling for this election. So I do not know how plausible this scenario is. But it is something to watch as the results come in later. This is not an mixed-member proportional (MMP) system, so coordination matters. And the left does not appear well coordinated in this election, a factor that should benefit the PRI, the president’s party, more than the PAN.

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* Sent by a contact in Mexico.

** The basic feature is that seats are allocated in parallel (i.e. based on nationwide proportional share of the 200 list seats) up to the point at which a party has an over-representation that is more than eight percentage points. Once a party has hit that cap, any remaining list seats are allocated proportionally among parties that have not hit the cap. There is a 2% threshold. Voters have just one vote, so parties must have candidates (their own or an alliance partner’s) in the single-seat districts to collect “list” votes, which are simply aggregations of parties’s candidates’ votes. (It is a bit more complex than this, but these are the main points.)