Villavicencio, also a journalist who had promised to fight corruption and cartels, was shot dead on August 9 in connection with a campaign meeting in the capital, Quito.
Earlier, Villavicencio’s party Construye said he should be replaced by vice-presidential candidate Andrea González. But the party was afraid that her candidacy could be rejected as she was already registered as a candidate for the post of vice president.
In a sign of the anxiety that now pervades the campaign, both Zurita and another candidate, Daniel Noboa, appeared on Sunday night wearing bulletproof vests before a televised debate.
Six people have so far been arrested and another killed in connection with the police investigation into the murder.
Ecuadorian President Guillermo Lasso has blamed the killing on organized crime.