After each election, regardless of the country, analyses draw maps of voting behavior and show how the sum of diverse trends in the different regions make up the final result at the national level. However, in very few cases, provinces or regions are observed that, due to the composition of their population, reflect national political trends; these territories are known as bellwethers. Whether in the state of Minas Gerais in Brazil, in the state of Ohio in the United States, or in the Autonomous Community of Aragon in Spain, election after election the election results are a mirror image of the outcome at the national level.

According to experts Laura Tejedor and José Luis Dader, bellwethers can be defined as cities, provinces, or regions that set or lead the national trend, although they can also be an almost exact copy of the national result. Therefore, bellwethers can also be a tool to predict the winner of a national election.

Based on what experts Edward Tufte and Richard Sun state in their book Are there Bellwether Electoral Districts? bellwethers are divided into two main categories. On the one hand, barometric districts predict the proportion of the national vote that the winner will receive, and on the other, swing districts detect changes or oscillations in the proportion of votes from one election to another. These two methods make it possible to measure the deviations in the regions from the national average and, of course, the number of times that the regions are correct a posteriori in voting for the winner of the elections.

Brazil and Minas Gerais

The recent presidential elections in Brazil reaffirmed that Minas Gerais is a fairly reliable bellwether state. In a previous analysis, we noted that from 1989 to 2022, the only federal state in Brazil that fit the definitions was Minas Gerais. It has been the only state, along with Amazonas, that has not failed in any of the electoral cycles to the presidency of that country and its indicators are almost exact to those of the national voting average.

During the first round, the barometric analysis showed that the vote received by Luiz Inácio Lula da Silva in Minas Gerais was only 0.14% below the national average. Furthermore, its average deviation since 1989 is 3.53 percentage points. As for the swingometer analysis, the differences in votes from one election to another in that state range from 3.16% to 0.80%.

This analysis allowed us to make estimates for the second round. We predicted that the result would be very close and, in the end, it was 50.90% of the vote for Lula and 49.10% for Jair Bolsonaro. The second round followed our previous analysis as observed in Minas Gerais. The percentage of the vote there was 0.70 points behind the national result and its variation between the first and second round was 0.56 points.

Spain and Aragon

In Spain, it is well known that what happens in Aragon is reflected on a national scale. Our analysis from 1977 to 2019 shows that this autonomous community is the only one that has always voted for the winner of the general elections. Moreover, if we analyze the provinces that compose it, we see that in general elections, Huesca and Zaragoza have always voted for the electoral winner. Meanwhile, the barometric analysis indicates that Zaragoza is the province that best reflects the percentage of voting for the winner of the general elections.

Another interesting type of analysis is found in the regional elections as predictors of what will happen in the country. Aragon is once again key. Despite the fact that the general and regional elections have not coincided in the same years until 2011, we can see what happens a year earlier in Aragon to understand the changes at the national level.

Our analysis establishes that the autonomic elections there are a good swingometer indicator, since 2011, the polls have picked up the same vote variations that would occur in the general elections months later. And the barometric analysis has allowed us to see that the percentage of the vote obtained by the most voted party in that entity is practically identical to the percentage of the vote obtained by the winner of the general elections at the national level.

Bellwethers as predictors: one explanation

Authors such as Edwards Tufte have explained in their research why bellwethers may be able to predict the results on a national scale. One of the most widely recognized possibilities is the existence of a population that is highly representative of the national picture. In this way, we could explain why the composition of their population evidences national political trends on a regional scale.

The sociological study of the population by strata can determine the similarities between the populations of these regions and the general population of the country according to parameters such as the proportion between men and women or the composition of the population by age. In addition to these similarities, there are others such as socioeconomic status and political preferences, which are conducive to electoral success election after election, whether in Ohio, Aragon, or Minas Gerais.

*Translated from Spanish by Janaína Ruviaro da Silva