MIDDLETON, Wis. — The irony was lost on no one. Senator Russ Feingold, a liberal with a fierce streak of independence who crusaded against the influence of money in politics, was toppled Tuesday in a campaign awash in the kind of unregulated cash he had struggled to keep out of the system.
And in a poignant twist, the loss came, in part, because independents flocked to his opponent, despite Mr. Feingold’s record of one maverick vote after another.
He was the sole senator to oppose the USA Patriot Act after the terrorist attacks on Sept. 11, 2001. He also broke with President Obama on several occasions, opposing the expansion of the war in Afghanistan, the bailing out of financial institutions in 2008 and the regulation of Wall Street this year, saying the restrictions did not go far enough.
Most prominently, he battled his colleagues to overhaul the campaign finance system; the resulting law, passed in 2002, bore his name and that of Senator John McCain, the Arizona Republican (who won re-election Tuesday).
After being eroded for years, the McCain-Feingold Act was gutted this year by the Supreme Court, helping to pave the way for millions of dollars to gush into campaigns from outside groups, most of whom do not have to reveal their donors — including at least $4 million in Wisconsin this year, virtually all of it against Mr. Feingold, 57, or for his opponent, Ron Johnson, 55, a wealthy Republican businessman.
Mr. Feingold rejected such money, as he had his entire career, but analysts said that probably had little to do with his loss.
“Independents deserted Democrats, period,” said Ken Goldstein, a political scientist at the University of Wisconsin at Madison. “This was not about Feingold’s record or the money or the advertising. It was about the anger of independents at the status quo.”
Still, others saw the flow of unregulated money as an added dimension to the narrative, in which Mr. Feingold was “hoist on his own petard,” said Mordecai Lee, who was first elected to the State Senate with Mr. Feingold in 1982 and is now a political scientist at the University of Wisconsin at Milwaukee.
“Because his good-government streak and his push for changes in the campaign finance system had no political constituency,” Mr. Lee said, “they led to the lawsuit that opened up the floodgates.”
As it happens, Mr. Feingold raised and spent more money than Mr. Johnson, at least as of mid-October. In fact, their arms race led to what appeared to be the most expensive Senate race in Wisconsin history, topping out at more than $35 million.
Mr. Feingold had raised $18.2 million and spent $16.2 million by the middle of last month, according to the Center for Responsive Politics. Mr. Johnson raised $12.8 million, and spent $10.5 million, pumping in more than $8.2 million of his own money.
Mr. Feingold has served in the Senate for 18 years and was seeking his fourth term. Mr. Johnson, a plastics manufacturer who had never run for office, won with 52 percent of the vote, to Mr. Feingold’s 47 percent.
Mr. Feingold was caught in an avalanche that crushed Democrats nationwide. Apart from capturing Mr. Feingold’s seat, Republicans here made a clean sweep of state government, winning the governor’s office and control of both houses of the State Legislature. One poll found that Mr. Obama’s approval rating here had declined at a faster rate than in any other state.
Despite his independence, Mr. Feingold allied himself strongly with certain Obama policies, including the health care bill, for which Mr. Johnson repeatedly bashed him.
Mike Wittenwyler, a Madison lawyer who had worked for Mr. Feingold in previous elections, said that the desire for “change” this year had overwhelmed everything else.
“This is the kind of climate where you would vote your mother out of office,” he said. “If you had a ‘D’ after your name, it was a liability.”
Mr. Feingold has spent his life in government. After winning a Rhodes Scholarship and graduating from Harvard Law School, he worked for a few years in private practice, then ran for the Wisconsin State Senate and has served in public office for nearly three decades.
It is not clear what he will do next. In a brief speech Tuesday night, he told the 300 supporters gathered in a hotel here in his hometown, “It’s on to the next fight, it’s on to the next battle, it’s on to 2012 and it is on to our next adventure. Forward.”
He then raised his fist in the air and left the stage.
A version of this article appeared in print on November 5, 2010, on page A21 of the New York edition.