DETROIT -- The Obama campaign continues to hammer the argument that it bailed out the U.S. auto industry here when Mitt Romney was willing to let it go through a bankruptcy.

On his third trip to the Motor City in recent weeks, Vice President Biden dropped in on the Metro Detroit AFL-CIO Labor Day Rally to again slam Romney's stance on the auto bailout.

(Carolyn Kaster/AP)

"Folks, you can't say you're going to create jobs in the United States of American when you're willing to let 1 million jobs go under with the liquidation of the automobile plants that he suggested," Biden told about 3,500 union workers, many in Tigers ball caps, firefighter helmets and American Federation of Teachers t-shirts, on a downtown street corner here.

Labor unions are a key part of Obama's campaign for working class votes and Biden heaped praise on them while going after Romney, a Michigan native son. He reminded the workers of the Republican's opposition to the pro-labor union law known as "card check," which hasn't gone anywhere on Capitol Hill, and told them: "We know who built this country and we know who is going to re-build it. It's you. And instead of vilifying you -- we should be thanking you. We owe you!"

"You're the reason that the automobile industry is back. Whether it was the wage freezes, the plant closures. Folks -- your sacrifices -- you sacrificed to keep your companies open," Biden said. "You took a hit. Because of you, Chrysler at the Jefferson Norton Plant just added another 1,100 jobs. ... Because of you and your productivity, the combined auto companies have committed to invest another $23 billion dollars in expansion in America. Not the Bain way ... not in Mexico ... not in China ... not in Vietnam, but in Detroit, Cleveland ... Right here in America!"

"Vice President Biden claimed that Americans are better off than they were four years ago, directly contradicting what President Obama and his campaign surrogates have said," said Romney spokeswoman Amanda Henneberg. "The truth is that the middle class has been crushed in the Obama economy. Unemployment remains high, incomes have fallen, and gas prices have doubled."

On this day, Biden mostly left the job of lauding the Obama administration to others. Sen. Debbie Stabenow, who addressed the crowd before the vice president, pointed to the New York Times op-ed Romney wrote in 2008 headlined "Let Detroit Go Bankrupt." "Thank goodness President Barack Obama and Vice President Joe Biden didn't pay attention to that and stood with us.," she said. "We're focused on manufacturing and making things again in America!"

The union rally wrapped up the vice president's Labor Day weekend campaign swing, which also included Pennsylvania and Wisconsin -- all swing states that Obama won in 2008.

On the same morning, President Obama was having breakfast with three autoworkers in Ohio.

Updated at 2:56 p.m.