Colombians will vote in a pivotal presidential election Sunday, pitting the socialist policies of the nation’s first leftist leader against a return to the more conservative, security-focused approach that once dominated Colombian politics.

“This government has provided help to many groups,” says Otilia García, in her 70s, of President Gustavo Petro’s social policies, including overhauling Colombia’s labor laws, almost doubling the minimum wage, and bringing more financial support for retirees who don’t qualify for pensions. “But the lack of security is killing us.”

Colombian law bars reelection, but Mr. Petro has framed the vote as a referendum on his government, which disrupted Colombia’s political status quo.…