Former President Donald J. Trump has taken his new obsession with the large crowds that Vice President Kamala Harris is drawing at her rallies to new heights, falsely declaring in a series of social media posts on Sunday that she had used artificial intelligence to create images and videos of fake crowds.

The crowds at Ms. Harris’s events, including one in Detroit outside an airplane hangar, were witnessed by thousands of people and news outlets, including The New York Times, and the number of attendees claimed by her campaign is in line with what was visible on the ground. Mr. Trump falsely wrote on his social media site, Truth Social, that “there was nobody at the plane, and she ‘A.I.’d’ it.”

A spokesman for the Trump campaign did not respond to a request for comment.

Mr. Trump has struggled to find his political footing in the weeks since President Biden decided to step aside and Ms. Harris replaced him atop the Democratic ticket: Mr. Trump questioned Ms. Harris’s racial identity at a conference for Black journalists, he later attacked Brian Kemp, the popular Republican governor in the key swing state of Georgia, and he has seen new polling that puts him behind Ms. Harris in several key states.

The Harris campaign has begun to mock Mr. Trump for his frustration over her crowds, one of which, it said, topped 15,000 people at an event in the Phoenix area on Friday.

“It’s not as if anybody cares about crowd sizes or anything,” Gov. Tim Walz of Minnesota, Ms. Harris’s running mate, said to the crowd, receiving a loud cheer.

In his posts on Sunday, Mr. Trump drew parallels between his false claims of fake crowds and his false claims of fraud in the 2020 presidential election.