BUENOS AIRES - Chile has been Latin America's success story since the 1980's, boasting rapid economic growth, successful integration into the world economy, solid democratic institutions, an effective state bureaucracy, and low levels of corruption. By most standards, the country is far better off than the rest of the region.

Not surprisingly, Chileans have kept the ruling Christian Democrat-Socialist coalition (the Concertación ) in power for four consecutive terms since 1990, when democracy was restored after 17 years of General Augusto Pinochet's repressive military rule. On December 13, however, voters in Chile's presidential election are likely to make the Concertación candidate sweat.

The Concertación is running Christian Democrat Eduardo Frei, the son of a former president who was president himself from 1994 until 2000. His opponent is Sebastián Piñera, a tycoon, former senator, and presidential runner-up in 2006 who represents the main opposition forces - Piñera's moderately conservative Renovación Nacional (RN) and the more rightist Union Democrata Independiente (UDI).

So far, nothing new: the RN and the UDI - which differ mainly in their attitude towards the military government from which they emerged (the RN being the more self-critical) - have been the main challengers in all the previous elections.

But change is coming, in the form of a 36-year-old maverick with no traditional-party backing. Marco Enríquez-Ominami, with about 20% support in recent polls, is remarkably close to Frei (around 26%), and not terribly far from Piñera (roughly 38%). The young candidate's unexpected popular support is rooted within the ruling coalition: his adoptive father is a prominent Socialist senator (his biological father, a leader of the revolutionary left in the 1970's, was killed by Pinochet's political police).

Marco himself was elected deputy on the Socialist ticket, but he defected when the party denied him the chance to contest Frei's nomination in a primary. With this unique mix of insider knowledge and outsider status, and a freshness that both main candidates lack, Enríquez-Ominami has gone farther than most observers predicted.

Piñera's lead and the emergence of Enríquez-Ominami are probably expressions of the same phenomenon: fatigue with - and within - the Concertación . Despite its many successes since 1990, and the high popularity of the current president (Socialist Michelle Bachelet, Chile's first female chief executive), time is taking a toll.

In recent years, several groups have split from the incumbent parties. Many voters are following suit, drawn mainly to Enríquez-Ominami. Frei's lackluster numbers reflect his own weaknesses (he is notoriously uncharismatic and was the least popular of the four Concertación presidents), as well as the inevitable stress that two decades of uninterrupted rule has placed on the ruling coalition.

As things stand now, the right is likely to win the first round, but with less than 50%. Even a comfortable Piñera victory could be reversed on the January 17th run-off, as Frei and Enríquez-Ominami are essentially splitting the center-left vote.

Enríquez-Ominami has the popularity and momentum to displace Frei from the run-off, but he may have a harder time defeating Piñera. The two most likely scenarios, then, are government continuity or predictable change (the right has been expected to capture the presidency for years). But a third scenario - less predictable change following a victory by Enríquez-Ominami - should not be ruled out, as campaign advertisements will appear in the media only starting November 13.

In terms of Chile's major policies, however, continuity is almost certain to prevail, even if Enríquez-Ominami wins. The center-left coalition that defeated Pinochet (in a 1988 plebiscite and his presidential candidate in the 1989 election) has been wise enough to keep, and in some cases deepen, the sound free-market policies inherited from the military government.

Chile boasts orthodox fiscal and monetary management, a very open economy, and a dynamic private sector. It was also the first country in the world to adopt a fully private pension system, a policy experiment carried out by one of Pinochet's technocrats (who happened to be Piñera's brother, José) and largely kept in place since then.

These policies are popular with voters, and they are protected by powerful actors (for example, the strong export-oriented business sector) and by trade agreements with almost all the world's major economies and regional blocs. Unlike many Latin American countries, in which populist backlashes have followed the liberalizing reforms of the 1990's, the main challenger in Chile is an even more pro-market coalition.

Beyond the economy, there are, of course, many areas of disagreement. The Concertación has dealt prudently with the political legacies of the dictatorship, gradually rescinding the self-serving rules that Pinochet created to protect himself, the military, and the right, and making some progress in prosecuting human rights violators.

But many are not happy: the far left - not very strong these days but with an important political tradition - wants more and faster progress on prosecutions, while the recalcitrant right insists on treating Pinochet (who died in 2006) as a national hero. Moral, ethnic, and environmental issues follow similar ideological lines. And yet the range of the policy debate is much narrower, and the tone more amiable, than is the case in Chile's more polarized neighbors, such as Argentina, Bolivia, and Venezuela.

In either of the most probable scenarios, continuity or prudent change, or even if the "outsider" candidate wins, Chile will in all likelihood remain a beacon of democratic stability, economic dynamism, and international engagement in a region too often characterized by political and economic turbulence.