I have been traveling to Mexico. It’s election time!
The midterm elections were held on Sunday. The results are not yet in, but we have enough data to make a prediction.
Now, this is just our prediction here at TPTM: there is a strong possibility that the Green Party will win fewer seats. But it may be useful to explain why the final seat counts will not be known until a few days after the election.
Mexico’s lower house uses a mixed FPTP-P.R. system. 300 seats are allocated on the basis of FPTP geographical districts. An addition 200 seats are allocated by proportional representation. The initial apportionment of those 200 seats does not depend on the apportionment of the geographical districts: if your party gets 10% of the vote, you get 20 seats plus whatever districts you won.
So far, so simple. But there are three wrinkles. The first is that no party can use its P.R. seats to obtain a higher proportion of seats than its vote share + 8%. In other words, if a party wins 200 district seats with 32% of the vote, then it receives no P.R. seats: 200 being 40% of 500. (That said, a party can exceed the vote-share + 8% limit if it wins enough districts; those seats are never taken away.)
The second is that parties can campaign in coalitions. The seats then have to be divided up inside the coalition. The PRI-Green coalition, for example, put up candidates in 250 districts: 192 from the PRI and 58 from the Greens. How many district seats go to which party makes a difference: the PRI cannot win more than 194 seats. (Once you throw out votes for independent candidates or parties that did not clear the 2% threshold, the PRI got 30.9% of the vote. 30.9% + 8% = 38.9%; 38.9% × 500 = 194.) Therefore it would be in the PRI’s interest to insure that the Greens get as many seats as possible.
Our estimate above relies on an unofficial count giving the PRI 29.1% and the Greens 7.06% of the vote; it also assumes that the coalition split its district seats proportionate to each party’s share of the total candidates: e.g. out of the 158 district seats won by the coalition, the estimate allocates 122 seats to the PRI and 36 to the Greens.
The final wrinkle is that for the purposes of proportional representation Mexico is divided into five 40-member electoral districts of unequal size, while our calculations assume one big national district. You can read about the system (in English) from the Mexican authorities here.
All that said, we should be pretty close. The upshot? The PRI does not have an outright majority, but it does in coalition with the Greens and Panal. A good result for President Peña. For Mexico, more mixed. More later, if there is interest, and quite possibly even if there is not!