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Mexican voting system explained, and shown if implemented in the US (Warning: contains pictures of spreadsheets)

Mexico will have its Mid Term election in less than a year. During this week, one of the polling houses in the country released a poll showing that the current majority party, Morena would increase their number of seats in the lower chamber of congress from 250 to 301 from a total of 500; an astounding achievement for the party.

However, Morena is only expected to attract 42% of the voter preference. So, why do they get more than 60% of congressional seats? Well, because voting systems are flawed. If the current system seems unfair, just consider that if Mexico used a First Past the Post system, Morena would be expected to earn 72% of the seats with just 42% of the vote.

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Mexico intended to make elections more representative by using a version of the Mixed Member Proportional system used in Germany. After a few modifications, it was named plurinominal system and put into place.

In Mexico’s plurinominal system, there are only 300 electoral districts across the country, but there are 500 seats available, that’s because 200 seats are not contested in regular elections, but through percentage share of the national popular vote.

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Those 200 seats, the plurinominal seats, are given to political parties to fill at their whim based on how many votes they got in the general election. As an example, if PAN got 20% of the popular vote and won 50 seats, they’d be allowed to have an additional 40 legislators, meaning their strength in the congress would be increased.

Parties can place any of their members on a plurinominal seat regardless of how popular or experienced this would-be legislator is. As a consequence, plurinominal assignments have been used with nepotistic ends in the past, like giving powerful positions to party insiders, or granting political immunity to friends of party leaders who might’ve done something illegal in the past. There is a catch though, if a political party gets less than 3% of the popular vote, it does not get any plurinominal seats awarded to it.

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Illustration for article titled Mexican voting system explained, and shown if implemented in the US (Warning: contains pictures of spreadsheets)
Screenshot: TAE

Using the polling house’s estimates, I created a spreadsheet showing the effects of the plurinominal representation system in the Mexican lower chamber of congress. As is clear, the two largest parties are overrepresented. But, thanks to the plurinominal system, the representation error (the sum of individual representation errors) goes down from almost 72% to 46%, though quite an improvement, said representation error is still higher than the one that resulted from the 2018 election, which was closer to 35%. The 2021 election, however, is going to be unusually binary, with just two parties, PAN and Morena, able to get any seats through direct election.

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Though the Mexican system has its limitations, both in the 2018 and in 2021 examples it reduces the power of the majority power and made it more representative of how voters wanted the country to be run. However, this system rewards political parties and punishes independent candidates, and individual candidates even inside a political party.

A big factor reducing possible misrepresentation in Mexican politics is the system of political alliances, in which small parties support a candidate from a larger one. In 2018 the largest parties (Morena, PAN, and PRI) formed political alliances with smaller parties to boost their numbers:

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Illustration for article titled Mexican voting system explained, and shown if implemented in the US (Warning: contains pictures of spreadsheets)
Screenshot: TAE

As seen both in 2018 and in 2021, the representation error is reduced when political alliances are considered. If parties form alliances it allows them to form their own identity but also let voters know that they can get behind another party’s nominee in particular electoral races. This is also sometimes abused, for instance the Green party in Mexico (PVEM) is always allied with the President’s party, as such in the last 20 years it has changed ideological allies three times.

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The biggest issue with the plurinominal system, however, is that it becomes more accurate as more parties overtake the 3% threshold, but it would have a very limited effect on the congress if there only were two parties. It also requires that more seats be renewed at the same time, effectively making the election more “nationally” based than “locally” based.

What I mean with that is, in elections candidates would be encouraged to discuss national issues as a unified party strategy rather than trying to get in touch with constituents. Even though plurinominal seats are divided into five “districts” and could potentially be assigned in phases, all seats are assigned in a single election every three years.

To show the effects of plurinominal representation in a country with two big parties, I prepared a would-be election result for the US house of representatives using the voter intention in the 2016 Presidential election and the makeup of the US House of Representatives in late 2019. The formula remains the same one used in Mexico:

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Illustration for article titled Mexican voting system explained, and shown if implemented in the US (Warning: contains pictures of spreadsheets)
Screenshot: TAE

The reason why I use the 2016 Presidential election as key rather than the 2018 midterm (where the power shifted from Republican to Democratic) is because I wanted to consider 3rd parties.

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In this exercise, every group except independents have their representation error reduced or kept the same, in particular, the largest party become less powerful. For the misrepresentation to go down, the minimum share of the national vote required would have to be reduced to 1%, as shown in this example:

Illustration for article titled Mexican voting system explained, and shown if implemented in the US (Warning: contains pictures of spreadsheets)
Screenshot: TAE
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In this example, once again, the representation error goes down. However, it increases when political alliances are taken into consideration.

Applying a different formula to the one used by Mexico can reduce the representation error further:

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Using this formula, it’s clear that considering a plurinominal system is useful to represent political groups more adequately in either nation. Eventually, if applied in the US, it would be able to increase the amount of political parties. Possibly like this:

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Illustration for article titled Mexican voting system explained, and shown if implemented in the US (Warning: contains pictures of spreadsheets)
Screenshot: TAE

In the end, more parties means more places for policy to be discussed, and more points of views to be considered. On this particular example, no one has even a close majority of the house, just the Moderate Conservative alliance. Even considering Republicans have, recently, been overrepresented thanks to the geography of the US.

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