Establishment conservatives quickly aligned with Hernández after his unexpected second-place win.

“Today the country of politicking and corruption lost,” Rodolfo Hernández, a right-wing anti-establishment candidate, said in a video message to his supporters, following the first-round election results in May that put him in second place behind Gustavo Petro.

Mr. Hernández’s unexpected second-place victory underscored the anti-incumbent fervor that has swept through the country and left it hungry to elect anyone who is not represented by the country’s mainstream conservative leaders.

In the months before the election, most of the country’s most powerful conservative politicians, and much of the business community, had lined up behind Federico Gutiérrez, the candidate of the conservative establishment.

But just minutes after Mr. Hernández’s win over Mr. Gutiérrez was solidified, key members of the dominant political class began to throw their support behind him for the runoff election.

“Rodolfo’s triumph is the triumph against the establishment,” María Fernanda Cabal, an influential right-wing senator whose husband leads a powerful cattle industry association, said on Twitter. “The country needs changes, not the suicide that Petro offers, but authority, order and the prosperity.”

Once it was clear that Mr. Hernández was the second-place winner, Mr. Gutiérrez said he would support him, a move that is likely to hand many of Mr. Gutiérrez’s five million votes to Mr. Hernández.

At a business complex in Bogotá, surrounded by supporters, Mr. Gutiérrez called his decision an effort to “safeguard democracy and safeguard freedom.”

“We don’t want to lose the country,” he said.

Mr. Gutiérrez was never expected to back Mr. Petro, an ideological opponent. But it wasn’t clear if he would support Mr. Hernández.

The announcement presents a major challenge to Mr. Petro, who some political analysts believe has hit his ceiling in terms of voters, and it could effectively hand the presidency to Mr. Hernández, a wild-card candidate who was largely unknown in most of Colombia just a few months ago.

Megan Janetsky contributed reporting from Bogotá.