But Mr. Uribe later split with Mr. Santos after his successor surprised the nation by starting peace talks with the country’s largest rebel group, the Revolutionary Armed Forces of Colombia, or FARC, saying he wanted to end decades of war. Mr. Uribe saw it as a betrayal, and many expected that Mr. Zuluaga would cut off the peace talks if he became president, and start a new military offensive. They also feared a Zuluaga victory would essentially mean a return to power by the polarizing Mr. Uribe.

Some on the left, including other leaders in Ms. López’s party, wanted to stay on the sidelines in the second round of voting, urging their supporters to check a blank box on their ballots as a protest against the two right-wing candidates.

But Ms. López decided to energetically endorse Mr. Santos. She recorded a television ad and appeared with Mr. Santos at campaign events, drawing harsh criticism from some in her own party who accused her of selling out.

“I dedicated myself to promote the vote for peace,” she said in an interview at her home here, decorated with paintings by famous Colombian artists. “No society can function if it is permanently in confrontation with itself, to the point where rifles are used to defend political positions.”

She said that for decades in Colombia, it was too easy to justify repression of the left by equating it with the guerrilla groups. That pattern, she hopes, will end with a peace deal, creating new opportunities for left-wing candidates like herself.

Jorge Restrepo, director of the Conflict Analysis Resource Center, a research institution, said that Ms. López occupied a unique place in Colombian politics.

“She has entry into the working-class unions, but also the same kinds of acceptance in high-level circles of the Colombian elite,” he said. “That’s precisely what makes her very interesting and defines her as someone who can get votes in a wide spectrum of Colombian electoral constituencies.”