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FLINT, Mich. — Senator Bernie Sanders of Vermont, in an impromptu news conference before the Democratic debate here, announced that he had acquired the endorsement of Donald W. Riegle Jr., a former Michigan senator, and then got a head start on attacking Hillary Clinton.

Mr. Riegle, a native of this city that is now reeling from a crisis over lead-tainted water, said he supported Mr. Sanders and launched into attacks on the trustworthiness and electability of Mrs. Clinton and the policies of her husband’s administration, which he said had “damaged” his hometown.

“The most important fact about Bernie is that he is honest and trustworthy,” Mr. Riegle said. “That’s an essential differentiator for me.”

As Mr. Sanders looked on stoically, Mr. Riegle, who retired from the senate in 1995 and has worked as a corporate lobbyist, said Mr. Sanders was the only Democratic candidate who could defeat the leading Republican candidates, Donald J. Trump and Senator Ted Cruz of Texas, in a general election.

“Her numbers don’t show that,” Mr. Riegle said of Mrs. Clinton. “This is the guy,” he adding, “It may seem improbable.”

Ahead of a debate that will be heavily focused on jobs and trade, Mr. Riegle delivered an indictment of the deregulation and trade policies of the Bill Clinton Administration.

“The Clintons rammed N.A.F.T.A. down the throats of the American people with false promises,” he said, in reference to the North Atlantic Free Trade Agreement. “I think the Clintons as a team need to be held accountable.”

Mrs. Clinton has long said she would renegotiate the three-nation trade deal Mr. Clinton signed into law in 1993. “I won’t support any agreement unless it helps create good jobs and higher wages for American workers,” she said of trade deals on Friday.

The endorsement from Mr. Riegle came as Mr. Sanders lags in the race for delegates behind Mrs. Clinton and polls in Michigan show him trailing the former secretary of state.