BALTIMORE (Reuters) - United Auto Workers President Shawn Fain on Friday criticized Donald Trump's U.S. presidential candidacy and praised President Joe Biden without ever using his name or repeating the union's January endorsement for his re-election this fall.

"It's clear that Donald Trump in the White House would be a complete disaster for the working class," Fain said at the Netroots Nation conference in Baltimore. Fain criticized Trump by name several times during his 16-minute speech that focused on the working class.

Fain then praised Biden without naming the president whose shaky performance in a debate last month had some fellow Democrats calling for him to drop his re-election bid.

"We've got a president in the White House who wants to stand with the working class and we've made incredible gains over the past 3-1/2 years with this president," Fain said.

The UAW's backing of Biden came into question after Reuters reported, citing sources, that the union's executive board late on Thursday met to discuss concerns with Biden's ability to defeat Trump in the November election.

The sources said the UAW was considering its next steps, but another source familiar with the union's thinking said rescinding its endorsement of Biden is not under consideration.

Fain and the UAW, which endorsed Biden in January, are important allies of the president and are expected to play a critical role as Biden campaigns to win key swing states including Michigan, where the UAW is based and many of its members live and work.

Asked in a Thursday evening news conference about the UAW's reported concerns, Biden replied that the union had endorsed him for president.

Biden and Trump have both made several appearances in Michigan to woo voters, especially union workers employed by U.S. automakers like Ford Motor ( F ), General Motors ( GM ) and Jeep-maker Stellantis ( STLA ).

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.

On Wednesday, Biden met with the executive council of the AFL-CIO, the largest federation of U.S. trade unions, to shore up support. Fain attended, and the AFL-CIO's executive council unanimously reaffirmed its commitment to re-electing Biden.

AFL-CIO President Liz Shuler said on Friday that U.S. unions were united behind Biden, adding the president walks the picket line with workers whereas Trump crosses them.

"We are standing strong with President Biden and Vice President Harris because they've stood strong with us," she told reporters on Air Force One. "I think UAW members know the difference between these two candidates."

Fain and the UAW backed Biden in January 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.

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