Chileans vote Sunday in presidential and congressional elections. Polls show that there will likely be a runoff required for the presidency, but the likely winner will be Michelle Bachelet of the Concertacion alliance. It is not clear which of the two candidates of the right will qualify for the runoff. Sebastian Piñera of the Renovacion Nacional and Joaquin Lavi­n of the Union Democrática Independiente are running within a few percentage points of each other for second place. Because Bachelet’s ultimate victory is so widely expected, the most interesting feature of this election is that, even with two presidential candidates on the right, their parties are running in alliance in the congressional races. To the best of my knowledge, this is the first time anywhere that two (or more) parties have presented an alliance for legislative races but not for president (normally one might expect the reverse, if not the same alliance patterns for the two branches).

Bachelet would be the first woman to serve as Chile’s president, and her victory would represent a second successive term for the candidate of the Socialist party. (Chilean presidents are not eligible for immediate reelection, so popular incumbent Ricardo Lagos is not running.)

Bachelet would probably defeat either conservative candidate, but Piñera would be more likely than Lavín to appeal to some Christian Democratic voters put off by once again having the Concertación represented on the presidential ticket by a Socialist. So, if there is a potential upset in the making, it is more likely if Piñera places second on Sunday than if Lavín does.

The presidential election marks the fourth since the end of the military dictatorship of Augusto Pinochet. Each of these elections has been won by the same electoral alliance, the Concertación, which consists of the Christian Democratic party (which supplied the first two presidents upon the return to democracy), the Socialists, and other center-left parties. This is the first time that presidential and congressional elections have occurred on the same day since 1993.

To understand why the center-left parties remain in an alliance despite the two-round election of the presidency (which would allow them to run separately in the first round and then pool votes in the runoff, as French alliances do), one must understand the congressional electoral system. There is no other electoral system like it in the world, and it puts a premium on electoral cooperation. It thus has helped sustain two large electoral alliances, despite internal tensions within them.

In fact, it is the very strength of the incentives for cooperation in Chile’s electoral system that explains why the center-right alliance has continued to present joint lists for congress even though its component parties failed to coordinate on a single presidential candidates (and are thus hoping to be able to pool their votes on one candidate to defeat Bachelet in the runoff).

The congressional electoral system, in both houses, is one of two-seat districts and open lists. The voter picks one candidate (in each house race) who is running on a list that also contains one other candidate nominated by the same alliance. The allocation rule used to determine how many seats each list wins in a district is the most common of all proportional-representation formulas, the d’Hondt divisor method. However, due to the small districts, the outcomes are not very proportional: Only two parties, at most, can obtain representation in any district, and the second list is likely to be over-represented. (The latter effect was intended by Pinochet, who discovered to his chagrin in 1988 that the right was not a majority if the Christian Democrats would continue to cooperate with the Socialists).

Under d’Hondt, the list with the largest number of votes wins the first seat (of course), and then its votes are divided by two. If the resulting quotient is still larger than the total votes cast for the second list, then the first list also wins the next seat. If not, then the second list wins the second seat. (In Chile, the process stops here, because there are only two seats per district, but if the magnitude were larger, the list that won the second seat would have its votes divided by three, and the process would continue in this fashion, always dividing a list’s votes by the number of the seat it just won in sequence, plus one, and then seeing which party now has the largest quotient at each step.)

In Chile’s two-seat districts, the result of this rule is that a list wins both seats in a district if, and only if, it doubles the votes of the runner-up list. So, you can see the reason for the premium on alliance. If the center-left were to present a joint list, but the right presented two separate lists for each of its two main parties, then in many districts the Concertación would win both seats and the right would win none.

As long as the right presents an alliance list, as does the Concertación, in all but the most one-sided districts, one seat will go to the center-left and one to the right. This means that the congressional outcome is relatively insensitive to changes in the balance of support between left and right. Only a very small number of districts in any election are at the cusp of one alliance winning two seats instead of one (or vice versa).

As I noted, the lists are open. That is, voters select a candidate within a list. If a list wins one seat, it is won by the candidate who won the greater number of votes of the two on that list. Because parties have a reasonably good idea of the districts in which their support is greatest, when the leaders of the parties comprising an alliance are selecting candidates, they can roughly determine the balance of the component parties in congress by nominating, for example, a strong Socialist on the same list with a weak Christian Democrat in one district, while doing the opposite in another.

So, the d’Hondt rule in two-seat districts and the ability of parties to divide up the nominations within their respective alliances means that competition in congressional elections is somewhat limited.

However, this year there is real competition on the right. Not only are the two main conservative parties presenting separate presidential candidates, but they also have some of their better congressional candidates competing against one another in districts in which only one of them is likely to win. Naturally, each presidential candidate wanted to be associated with strong co-partisan legislative candidates wherever possible.

Barring an upset, the outcome of this election—another Socialist presidency within the Concertación, and not much change in the left-right balance in congress—is a foregone conclusion. But the competition on the right will be interesting to watch for what it reveals about the relative strength of two parties that are now trying to distinguish themselves more than ever.

That two presidential candidates of the same alliance are nonetheless pooling their votes in congressional races shows the imperative the Chilean electoral system puts on cooperation, however strained it may be at times.

It is likely that the two-seat districts will be abolished in the near future. A return to PR in larger districts would allow the reemergence of the perhaps more-natural tendency in Chile of the Christian Democrats and moderate right to work together. It would also allow the extra-Concertación left (whose candidate could win 7% or more on Sunday) to gain representation. Now that the years that have elapsed since the end of the Pinochet dictatorship are longer than that dictatorship itself, Sunday’s election could mark the beginning of the end of the two big alliances that first formed over the question of Pinochet’s continuismo.