The posters dotting the streets of Buenos Aires had a certain Soviet flare to them.

There was one of Argentina’s presidential candidates, Sergio Massa, dressed in a shirt with what appeared to be military medals, pointing to a blue sky. He was surrounded by hundreds of older people — in drab clothing, with serious, and often disfigured, faces — looked toward him in hope.

The style was no mistake. The illustrator had been given clear instructions.

“Sovietic Political propaganda poster illustration by Gustav Klutsis featuring a leader, masssa, standing firmly,” said a prompt that Mr. Massa’s campaign fed into an artificial-intelligence program to produce the image. “Symbols of unity and power fill the environment,” the prompt continued. “The image exudes authority and determination.”

Javier Milei, the other candidate in Sunday’s runoff election, has struck back by sharing what appear to be A.I. images depicting Mr. Massa as a Chinese communist leader and himself as a cuddly cartoon lion. They have been viewed more than 30 million times.

Argentina’s election has quickly become a testing ground for A.I. in campaigns, with the two candidates and their supporters employing the technology to doctor existing images and videos and create others from scratch.

A.I. has made candidates say things they did not, and put them in famous movies and memes. It has created campaign posters, and triggered debates over whether real videos are actually real.

A.I.’s prominent role in Argentina’s campaign and the political debate it has set off underscore the technology’s growing prevalence and show that, with its expanding power and falling cost, it is now likely to be a factor in many democratic elections around the globe.