(Adds Biden quotes, Trump spokesman in paragraphs 6-8; ; context)

WASHINGTON, Jan 24 (Reuters) - The United Auto Workers union's leader endorsed U.S. President Joe Biden's reelection bid on Wednesday with a fiery speech in Washington that was also harshly critical of Republican former President Donald Trump.

In a full-throated endorsement of the Democratic incumbent, UAW President Shawn Fain cited Biden's pro-union record and his decision to become the first president to join a union picket line this summer during a successful autoworkers strike for higher pay.

"Instead of talking trash about our union, Joe Biden stood with us," Fain said. Fain and Biden previously had sharp differences over electric vehicle policy, and the endorsement could be a strong boost to Biden in Michigan and other manufacturing states.

Fain heaped criticism on Trump, citing his failure to persuade General Motors to keep an assembly plant open in Ohio and his appearance at a non-union hall during last year's strike, and called him a "scab."

"Donald Trump stands against everything we stand for as a society," Fain said.

Biden took the floor to loud cheers and applause from hundreds of UAW members, telling them, "I was so damn proud to stand in that picket line with you."

Biden said Fain demonstrated "extraordinary leadership" during the UAW's historic strikes and called him a "leader with a backbone, a backbone like a ramrod."

Biden also vowed to push back against the outflow of manufacturing jobs to China and touted the passage of the Inflation Reduction Act, which is offering incentives for automakers and others to manufacture in the U.S.

Steven Cheug, a Trump campaign spokesperson, said: "President Trump represents the working men and women of America, and his America First policies helped protect American jobs from being either shipped away or terminated."

Biden has already pocketed endorsements from most of the nation's major labor unions. But the seal of approval from the autoworkers has taken much longer despite a full-court press that included Biden joining striking workers when they targeted the Detroit Three automakers in September.

Biden has strongly backed the UAW's efforts to unionize carmakers Tesla, Toyota and other automakers.

Biden narrowly won Michigan in 2020.

A Biden campaign official said this endorsement will mean more in November in Michigan than the anger among Muslim voters in the state over the administration's support for Israel.

The UAW won record-setting contracts last fall, including a 25% increase in base wages with General Motors, Ford and Stellantis. Fain has praised Biden administration officials for helping win those contracts and ensure battery plant workers get higher wages.

In May, Fain said the union was not ready to endorse Biden for a second four-year term, citing concerns at the time about the president's EV policies.

(Reporting By Jarrett Renshaw; additional reporting by David Shepardson; editing by Paul Thomasch and Jonathan Oatis and Heather Timmons)