DETROIT/WASHINGTON (Reuters) -United Auto Workers President Shawn Fain met with the union's executive board late on Thursday to discuss his deep concerns with President Joe Biden's ability to defeat Donald Trump in the November election, three sources familiar with the matter said.

Fain called together top officials at the nearly 400,000-member union to discuss concerns and what the union's options are, according to the sources, who asked not to be identified. The union is considering its next steps, the sources said.

Fain and the UAW, which endorsed Biden in January, are important allies of Biden and are expected to play a critical role in helping Biden win in key swing states including Michigan, where the UAW is based.

Biden and Trump have both made several appearances in Michigan to woo voters, especially those employed by U.S. automakers like Ford Motor, General Motors and Jeep-maker Stellantis.

UAW communications director Jonah Furman and the Biden campaign declined to comment.

Fain, who announced the UAW's endorsement of Biden in January, has repeatedly rebuked Trump, the Republican former president.

"Donald Trump stands against everything we stand for as a society," Fain said in his January endorsement of Biden.

On Wednesday, Biden met with the executive council of the AFL-CIO, the largest federation of U.S. trade unions, to shore up support among labor unions. Fain attended that meeting.

"I think of you as my domestic NATO-not a joke," he said. Biden also thanked union leaders for their support, and shared his plans for the future, seeking to cast aside doubts over his reelection campaign.

Fain backed Biden after the president joined a union picket line during a six-week autoworkers strike last year that resulted in historic wage increases for workers at the Detroit Three carmakers. Biden was the first U.S. president to walk with striking workers. Fain also received a mention from Biden at this year's State of the Union address, which Fain attended.

Biden, who has characterized himself as the most pro-union president in U.S. history, has recently leaned on key labor leaders to drum up support among his base.

Biden is facing a growing calls to step aside as the Democratic candidate after a shaky debate performance that has raised questions about his mental fitness for office. For nearly two weeks, 81-year-old Biden has sought to stem defections by Democratic lawmakers, donors and other allies worried he stands to lose to Trump, 78.

At least 12 U.S. congress Democrats have called on Biden to end his reelection bid so far. On Wednesday, actor George Clooney, a Democrat who has backed Biden, withdrew his support in an opinion piece in the New York Times.