On paper, Enrique Peñalosa looks like the best president Colombia might never have.

As mayor of Bogotá between 1998 and 2001, Peñalosa introduced the widely popular TransMilenio rapid bus system, expanded a sprawling network of bicycle paths, and generally built upon the foundation on the progress established by his predecessor, Antanas Mockus. Together, Mockus and Peñalosa arguably transformed Bogotá into one of the most developed urban spaces in Latin America.

Six weeks ago, Peñalosa seemed to have the momentum in Colombia’s presidential election, building on his reputation as a moderate, non-corrupt public official. Poll after poll showed him vaulting into second place and gaining ground against the incumbent, Juan Manuel Santos. Early in April, polls started showing that he was nearing 20% support and, more incredibly, that voters preferred Peñalosa to Santos in a hypothetical runoff.

That’s all before Óscar Iván Zuluaga, the conservative candidate allied with former president Álvaro Uribe, started gaining traction. With Colombians set to vote on Sunday in what will likely be the first of two rounds of their presidential election, Zuluaga is now tied with Santos, according to polls, and either one could win the June 15 runoff.

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RELATED: Five reasons why Zuluaga is beating Santos
in Colombia’s election

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Meanwhile, Peñalosa has fallen back to single digits in polls. No one gives him much of a chance to advance to a runoff — in contrast to Mockus, who finished second in the 2010 presidential election as the candidate of the Partido Verde Colombiano (Colombian Green Party). In that election, Santos, running with Uribe’s support and on his record as Uribe’s defense minister, easily dispatched Mockus by a margin of 69.1% to 27.5%, given Mockus’s leftist politics.

This time around, many voters otherwise inclined to support Peñalosa have instead lined up behind Santos to block Zuluaga’s election, which would almost certainly torpedo the Santos administration’s current negotiations to bring to an end the 50-year guerrilla insurgency of the Fuerzas Armadas Revolucionarias de Colombia (FARC, the Revolutionary Armed Forces of Colombia).

The FARC negotiations are the main reason for the split between Santos and Uribe, who formed his own party last year, Centro Democrático (Democratic Center) and won a seat in the Senado (Senate), capping off his new party’s impressive, if not overwhelming, performance in March’s congressional elections.

When Zuluaga was a minor candidate in the elections, voters saw Peñalosa as a credible challenger. Unlike Santos, Peñalosa isn’t part of the political machine of the Partido Liberal Colombiano (Colombian Liberal Party), and Peñalosa had tried to capitalize on voter disgust with mermelada, the practice of trading political support for allocated government funds for pet projects, not unlike earmarking in the United States. Voters also saw in Peñalosa a candidate who wouldn’t sabotage the Santos administration’s FARC efforts and who, as president, might nudge Colombia toward stronger social welfare policies without turning the economy into a socialist basketcase.

As Zuluaga became a real contender for the presidency, however, many of Peñalosa’s potential supporters have rallied behind Santos in order to save the best chance in decades to end the FARC insurgency. That holds true for some of the highest-ranking leaders within Peñalosa’s own camp.

Peñalosa, a member of the Green Party, is widely viewed as a much more pragmatic figure than Mockus. An economist by training, Peñalosa has a reputation as a business-friendly moderate who could command a broader base of supporters than Mockus in a presidential runoff. Peñalosa’s pragmatism is one of the reasons that he lost two bids, one in 2007 and another in 2011, for another term as mayor of Bogotá, the one place in Colombia where the electorate leans left. In 2011, when Peñalosa narrowly lost to his more progressive rival, Gustavo Petro, Mockus resigned in protest from the Green Party when Peñalosa accepted the conservative Uribe’s endorsement in the mayoral race.

Now, in the presidential race, Peñalosa is now running under the banner of the Alianza Verde (Green Alliance), whose rank-and-file supporters are much more leftist than Peñalosa himself.

Unfortunately for Peñalosa, it’s neither particularly green nor particularly allied.

The merger, between the Green Party and Petro’s Movimiento Progresistas (Progressive Movement), brought the two rivals from the 2011 mayoral race into what was always an awkward alliance. But far from lining up behind Peñalosa’s candidacy, Petro and Mockus (who still hasn’t returned to the Green Party), have urged supporters to back Santos in hopes of salvaging the FARC peace process.

If all of this is confusing, the bottom line may be that if Peñalosa is too conservative to return to Bogotá politics, he still seems too leftist to win a national election when  polls show Zuluaga is such a credible threat.

Many thanks to Robert Kelley — like Peñalosa, a fellow Duke alumnus — for directing me to several links and ideas in this post.