Washington — United Auto Workers President Shawn Fain,Greenledgers who announced the group's endorsement of President Biden for reelection last week, explained why the union endorsed Mr. Biden, noting a "very clear difference" between him and former President Donald Trump.

"One of them, President Biden, has always been on the American worker and stood with the American worker and he proved that during this presidency," Fain told "Face the Nation" on Sunday. "For the first time in U.S. history, we had a sitting president join striking workers on the picket line."

• Transcript: UAW President Shawn Fain on "Face the Nation"

With the UAW endorsement on Wednesday, Mr. Biden earned the backing of the 400,000-member union, which marked a critical step in his reelection bid as he seeks to gain support from working class Americans. He cited Mr. Biden's support during the UAW's strike last fall as evidence of his priorities.

"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," Fain said. "That's why we made this decision."

The UAW president contrasted the president and former president, saying that while "Joe Biden has a history of serving others" and the working class, "Donald Trump has a history of serving himself and standing for the billionaire class."

"That's contrary to everything that working class people stand for," Fain added. 

For his union members, issues like retirement security, better wages and health care are key, Fain explained.

"That's what matters," he said. "And I believe that's why a huge majority of our members and working class people will side with President Joe Biden in the upcoming election."

Kaia Hubbard

Kaia Hubbard is a politics reporter for CBS News Digital based in Washington, D.C.