Whoever wins, Colombia will inaugurate its first Black woman vice-president in August - Petro's running mate Francia Marquez and Hernandez's second-in-command Marelen Castillo are both Afro-Colombian.
Colombians have begun voting to elect their next president, choosing between a leftist former guerrilla who is pushing profound social change and an eccentric construction magnate who has promised to fight corruption.
The candidates in Sunday's poll are Gustavo Petro, once a member of the M-19 rebels, and Rodolfo Hernandez, who first gathered support through TikTok videos, are technically tied in polling.
Petro, a former mayor of capital Bogota and current senator, has pledged to fight inequality with free university education, pension reforms and high taxes on unproductive land.
His proposals — especially a ban on new oil projects — have startled some investors, though he has promised to respect current contracts.
Petro, who is making his third presidential bid, would be Colombia's first leftist president, adding the Andean nation to a list of Latin American countries which have elected progressives in recent years.
Petro, 62, said he was tortured by the military when he was detained for his involvement with the guerrillas and his potential victory has high-ranking armed forces officials bracing for change.
READ MORE: Colombians vote for president amid struggling economy
Police on maximum alert
Hernandez, who served as mayor of Bucaramanga, was a surprise contender in the run-off and has promised to shrink government and to finance social programs by stopping corruption.
Despite his anti-graft rhetoric, Hernandez is under a corruption investigation himself over allegations he intervened in a trash management tender to benefit a company his son lobbied for. He has denied wrongdoing.
Hernandez — like Petro — has pledged to fully implement a 2016 peace deal with FARC rebels and seek talks with the still-active ELN guerrillas, even though he accuses that group of kidnapping and murdering his daughter Juliana in 2004.
The ELN has denied any involvement and her body has never been found.
Hernandez has pledged to respect the results, while Petro has raised doubts about the integrity of electoral officials.
Both men have canceled campaign events because of what they said were threats to their lives.
Colombia's police said this week they are on maximum alert after detecting plans by radical groups to commit acts of violence connected to the vote.
Some 39 million Colombians are eligible to cast ballots.
Just under 55 percent of them turned out for the first round - which, like the second, is taking place over a holiday weekend.
READ MORE: Colombia leftist wins decisive presidential primary victory
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