Small Constituencies Matter (I): Why the Last Seat in a Four-Seat Province Is Worth So Much

Twenty-nine of the 52 constituencies elect five members or fewer and together fill 108 of the 350 seats. In them, the last seat is decided by fewer votes than anywhere else, often changes blocs, and weighs a third, a quarter or a fifth of the whole province. Of the 26 last seats decided by fewer than 1,000 votes since 2008, 17 were in constituencies of five seats or fewer.

24 October 2026 · Miguel · Leer en español

When people talk about Spain’s general elections, they talk about Madrid, Barcelona, Valencia or Seville, which elect between 12 and 38 members of the Congress of Deputies. But 29 of the 52 constituencies elect five or fewer, and together they fill 108 seats, almost a third of the chamber. In this first part we explain why the last seat in those provinces is so special; in the second, what is at stake in Cantabria, La Rioja, Navarre, Murcia and four other provinces on November 29.

How the last seat is allocated

Under the D’Hondt method, each party’s votes are divided by 1, 2, 3 and so on, and the seats go to the highest quotients. In a four-seat province, the fourth seat is the fourth-highest quotient: it can be the third seat of the leading party or the first seat of the third party. It is a comparison between two numbers that are usually close, and that is where the winner and the loser in that province are decided.

The legal 3% barrier almost never matters in these provinces. The one that does is another, which does not appear in the law.

The threshold nobody sees

The fewer seats a constituency has, the more votes a party needs to get in. A classic approximation from political science is the effective threshold, 75% divided by the number of seats plus one (Taagepera, 2002): below that percentage, it is rare for a party to win a seat.

This explains a good part of November 29. Outside the Basque Country and Navarre, the far-right Vox is at around 14–22% in almost all the small provinces: right around those thresholds. Frente Amplio, the left alliance succeeding Sumar, which is below 8% in many of them, is left out of almost all. One point up or down for Vox in a four-seat province is the difference between zero members and one.

Tighter in percentage in the big ones, cheaper in votes in the small ones

For each constituency and each election since 2008, we measured how many votes the closest rival was missing to take the last seat. The result has two sides.

In percentage terms, the last seat is tighter the bigger the province: in those with ten seats or more it was decided by less than one point in 61% of elections; in those with one to three seats, in only 8%. It makes sense: the more seats, the more parties are close to the last quotient.

In votes, though, it is the other way round. The last seat of a province with one to three members was decided by a median of about 5,400 votes; that of provinces with ten or more, by about 12,800. In the small ones, a few thousand votes move an entire seat. And in the extreme cases, fewer than a thousand: since 2008, 26 last seats were decided by fewer than 1,000 votes, and 17 of them were in constituencies of five seats or fewer.

(PP is the conservative People’s Party, PSOE the Socialists, and EH Bildu the Basque left.)

A seat that weighs a quarter of the province

There are three reasons why those seats matter more than they seem:

• Each seat is an enormous share of the province. In Madrid, the last seat is one of 38; in La Rioja, one of 4. A swing in La Rioja’s last seat changes 25% of its representation.
• They often decide between blocs. In the big provinces, the last seat is sometimes contested by two parties of the same bloc. In the small ones, with three or four parties in contention, it is common for the last seat to pass from the right to the left or vice versa. That is why they are where any change in the combined total of the PP and Vox, or of the PSOE and its partners, is most noticeable.
• There are many of them. Twenty-nine constituencies of five seats or fewer are 29 last seats in play, against only seven in those of ten or more. And since each vote in them weighs more than in the big ones (we explain it in what a vote is worth), a small shift in votes in the small ones is worth more seats than the same shift in Madrid.

What to expect on November 29

Our forecast confirms that the last seat is not easy anywhere. In provinces of four or five seats, the party with the best chance of taking the last one gets it in 51% of simulations on average; in those of one to three, in 59%; and in those of six or more, in only 37%. In other words: in the big ones there is more uncertainty about who takes the last seat, but in the small ones what is at stake is bigger, because it usually changes the split between blocs.

In the second part we go down to eight specific cases: Cantabria, La Rioja, Navarre, Murcia, Soria, Teruel, Álava and Huesca. And on our last seat page you will find all 52 constituencies, with each party’s probabilities.

How we calculated it

• Margins 2008–2023. For each constituency and election, with the official results from the BOE (Spain’s official gazette) and the D’Hondt allocation reproduced seat by seat: new votes the cheapest rival needed to take the last seat, without subtracting votes from anyone (multi_margins.py). In percentage, over the constituency’s valid votes.
• Size bands. By the number of seats in 2026 (Madrid 38 and Cádiz 8; the rest, as in 2023).
• Effective threshold. 75% / (seats + 1), an approximation by Rein Taagepera widely used in political science; it is not a legal barrier.
• Forecast. The simulations of today’s forecast: probability that each party takes the last seat in each constituency.
• Code. articulo_pequenas.py, in the altnalysis repository.