Javier Milei, a far-right libertarian economist, has stayed aloft in Argentina’s presidential campaign on the wings of the youth vote.

To win the runoff election this month, he will need to hold on to that key demographic, pollsters say. But now, a major hurdle stands in his way: Swifties.

Squadrons of Argentine fans of the pop star Taylor Swift have gotten political. They have trained their online sights on Mr. Milei and his rising libertarian party, framing them as a danger to Argentina, while Ms. Swift herself is preparing to arrive in Argentina next week for the launch of her Eras Tour outside North America.

“Milei=Trump,” said one post from a group called Swifties Against Freedom Advances, which is the name of Mr. Milei’s party.