President Trump is doubling down on his attacks against the president of the United Auto Workers, Shawn Fain, in the wake of the union’s endorsement of President Biden, as he seeks to chip away at support for Mr. Biden among union members.

In a post on Truth Social on Monday evening, Mr. Trump called Mr. Fain a “Weapon of Mass Destruction on Auto Workers and the Automobile Manufacturing Industry,” asking, “Is he under contract to China?”

“He is a real ‘STIFF’ who is selling the Automobile Industry right into the big, powerful, hands of China,” Mr. Trump said, adding that “the rest” of the auto manufacturing industry will go to Communist China “if I am not elected President.”

Mr. Trump’s attacks on Mr. Fain come after the union endorsed Mr. Biden for the presidency in 2024 and Mr. Fain called Mr. Trump a “scab” during the union’s conference at Washington, D.C. 

Mr. Fain responded to Mr. Trump’s comments on MSNBC, saying they show a “perfect contrast between the two candidates.”

“You have, for the first time in history, a sitting U.S. president joining working-class people, joining the workers on the picket line, standing up with them,” Mr. Fain said. Then there is Mr. Trump, Mr. Fain added, “who claims he supports the workers, who calls one of his business owner buddies in a non-union factory, and he goes to this non-union factory and has a rally claiming that he’s there for the union workers and the striking workers.”

The pitch from Mr. Trump — that Mr. Fain and Mr. Biden are colluding to send the American auto industry to China — is part of a larger strategy to circumnavigate union leadership and appeal directly to union members.

Mr. Trump’s strategy dates back to the beginning of the UAW’s strike in the fall, when he suggested that members should support him instead of Mr. Biden, citing tension between the union and the president over Mr. Biden’s support for electric vehicles and the union’s demand that electric vehicle plants be included under union bargaining agreements.

While Mr. Trump’s overtures to Mr. Fain appear to have fallen flat, he is courting other union leaders, like the president of the Teamsters, Sean O’Brien, who met with Mr. Trump at Mar-a-Lago earlier in January.

Mr. Trump has also brought his appeals directly to auto manufacturing employees, visiting a non-union plant during the UAW strike last year.

On “Face the Nation” Sunday, Mr. Fain said he thought that a smaller percentage of UAW members would vote for Mr. Trump in 2024 than did in 2016. Mr. Fain has previously estimated that about 28 percent of UAW members supported Mr. Trump in 2016.

“I believe the overwhelming majority of UAW members and working-class people, when the facts and the truth are put in front of them, will support Joe Biden for president,” Mr. Fain said. “I can’t fathom [that] any union would support Donald Trump for president.”

Although Mr. Trump is often credited with appealing to working-class Americans more than a typical Republican candidate, union members overall have drifted back toward Democratic candidates in recent years.

A recent Morning Consult survey found that, since 2017, the proportion of union members who align themselves with the Democratic Party has risen to 51 percent from 40 percent. 

At the same time, the proportion of union members who align themselves with the Republican Party has dropped to 23 percent from 30 percent.

A  Suffolk University poll from October found that support for Mr. Trump among union members slightly outpaces Republicans in general, with 31 percent of union members surveyed saying they planned to vote for Mr. Trump in 2024.

The UAW’s endorsement and efforts to get members to support Mr. Biden could influence the union’s more than 400,000 active members and about 580,000 retired members across America  — and potentially their families — to break more heavily for Mr. Biden than they did in the past.

A large number of these members, around 134,000 according to the union, are in Michigan, a key swing state in the 2024 presidential election. In 2016, Mr. Trump carried Michigan by fewer than 11,000 votes. In 2020, Mr. Biden won Michigan by about 154,000 votes.