Russ Feingold, the liberal Democrat and former three-term senator, announced on Thursday that he will run for his old seat in the United States Senate from Wisconsin.

He announced his comeback bid in a YouTube video from his home in Middleton, Wis.

“People tell me all the time that our politics and Washington are broken, and that multimillionaires, billionaires and big corporations are calling the shots,” Mr. Feingold said, before announcing his intention to run and pledging to “bring back to the U.S. Senate strong independence, bipartisanship and honesty.”

Mr. Feingold’s tenure in the Senate was highlighted by his battle against money in politics, like the Bipartisan Campaign Reform Act, commonly known as McCain-Feingold, which was passed in 2002, but weakened by recent Supreme Court decisions on campaign finance law.

Mr. Feingold’s announcement sets up a likely rematch with Senator Ron Johnson who defeated Mr. Feingold in 2010.