A populist outsider is shaking up Colombia’s presidential election in the final days of campaigning, opening up what had largely been a two-horse race between radical leftwing senator Gustavo Petro and Federico Gutiérrez of the centre-right.

Rodolfo Hernández, a 77-year-old businessman and anti-corruption crusader, has shot up in opinion polls and threatens to pip Gutiérrez to second place in next Sunday’s vote. If he does, he is likely to face Petro in a run-off in June — and polls suggest it would be a close fight.

“The Colombian people and I are the only ones who can beat Petro in the second round,” Hernández wrote on Twitter this week after three polls suggested he was gaining momentum and closing on Gutiérrez. “I will be your president.”