On Thursday, Mr. Cooney posted the video on Twitter, framing it as a “special message” for his opponent.
Mr. Christie responded by telling NJ Advance Media that he had been hoodwinked, and he tweeted about the incident, emphasizing that he was on Cameo for the benefit of a New Jersey-based charity.
“Shame on @CooneyforMT,” he said in his tweet, stating his support for Mr. Gianforte. (With five days to go, the Montana governor’s race is tight; in a poll released on Wednesday, Mr. Cooney and Mr. Gianforte were tied.)
The idea to prank Mr. Christie originated with Mr. Cooney’s campaign manager, Brad Elkins, who spotted the former governor on Cameo. The move was meant to emphasize a message that Mr. Cooney’s campaign has been pushing: that Mr. Gianforte — who was born in California and spent his early adulthood in New Jersey — is an outsider, with outsider values.
“It’s been a fun day,” said Matt Fidel, a campaign spokesman.
Other Trump associates have been susceptible to pranks like the one organized by Mr. Cooney’s staff. In the last several months, Rob Flaherty, the digital director of Joseph R. Biden Jr.’s presidential campaign, has commissioned Cameo videos from one of Mr. Trump’s former campaign managers, Corey Lewandowski, and Sebastian Gorka, a former White House adviser, as well as the president’s longtime friend Roger Stone.
Each video was made to pump up “Joey” (Mr. Biden) before his debates with Mr. Trump.
Mr. Flaherty tweeted the first clip, from Mr. Lewandowski, on Sept. 29; the one from Mr. Gorka, on Oct. 15; and the final one, from Mr. Stone, on Oct. 22. (The second debate was canceled after a disagreement over Covid-19 safety protocols.)