Colombian voters narrowly rejected that agreement in a plebiscite early this month, many of them swayed by a hyperbolic and misleading campaign led by Mr. Uribe. He and his allies accused Mr. Santos of offering blanket amnesty to Marxist war criminals whom he warned could end up taking power in the country. He also asserted, without evidence, that the deal would hurt the private sector. The politician who oversaw Mr. Uribe’s No campaign, Juan Carlos Vélez, even admitted in an interview that they had steered clear of explaining the content of the agreement and instead “focused on a message of indignation.”

This week, Mr. Santos announced that his government would soon formally start peace talks with the second-largest rebel group, the National Liberation Army, or ELN, in Ecuador.

In order for the FARC deal to get back on track, and for talks with the ELN to succeed, Mr. Uribe will need to play a constructive role. After the referendum, Mr. Uribe made a series of unrealistic demands on the peace deal with the FARC, including scrapping the transitional justice system with a special tribunal that was at the heart of the deal. That tribunal would offer amnesty to most rank-and-file fighters and leniently punish guerrilla members who confess to grave crimes.

If Mr. Uribe has a better, workable idea, he should dispatch a delegation to Havana, where the FARC leaders are currently based, to seek compromises on issues involving justice and political participation. If all sides are willing to negotiate in good faith, a final peace agreement could be reached before the end of the year. In recent days, thousands of Colombians who support the peace agreement have taken to the streets to call on the political establishment to work together toward a prompt resolution.

If the fight drags on beyond this year, international aid pledged for the implementation of the peace deal will very likely start to fizzle. The United Nations, for instance, has already sent teams of observers, who will monitor compliance with the agreement and take custody of the weapons of the rebel fighters. Those teams cannot be expected to wait around indefinitely for a political breakthrough.