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Independent political groups have swept decisively into Wisconsin's U.S. Senate race this year, with federal records showing that outside organizations have spent more than $21 million in the tightly contested election.

That's seven times more than the independent money spent in the 2010 Senate race.

For the public, it's been hard to ignore. The huge injection of cash has amplified the sharply negative tone in the race between Republican Tommy Thompson and Democrat Tammy Baldwin.

So far, figures show that almost twice as much money - mostly for television advertising - is being used against Thompson.

As of Saturday, Wisconsin ranked second in outside spending, underscoring the importance of the Badger State and a handful of other states as Republicans and Democrats wrestle for control of the Senate. Democrats hold a slim majority in the Senate.

According to the Federal Election Commission, the leading states in outside money are: Virginia, $23.7 million; Wisconsin $21.6 million; Ohio, $17.6 million; Florida, $15.4 million; and Texas, $15 million.

Most of the money in Wisconsin has been spent since the Aug. 14 primary.

By comparison, just $3 million came from independent groups in the 2010 race between incumbent Russ Feingold, a Democratic who lost to Republican Ron Johnson.

This year's outside money comes from an array of sources. Billionaire investor George Soros and Hollywood mogul Jeffrey Katzenberg are backing Democratic candidates through a group called Majority PAC. Another group, Crossroads GPS, was organized by Republican strategist Karl Rove and billionaire brothers David and Charles Koch are backing another big player, Americans for Prosperity.

The attention on Wisconsin shows that these powerful groups believe their spending can influence the outcome of the race, said Jennifer Duffy, senior editor of The Cook Political Report. Her organization continues to rank the race as a tossup.

Baldwin, a member of the U.S. House from Madison, and Thompson, the former governor, are locked in a tight election to replace Herb Kohl, a Democrat, who is retiring.

A poll by Quinnipiac University, The New York Times and CBS News released on Thursday showed Baldwin leading Thompson, 48% to 46%, with pollsters saying the race is too close to call. A poll released on Oct. 3 by Marquette University Law School had Baldwin up by 4 points.

Whether the outside spending will outpace the cash raised by Thompson and Baldwin isn't clear. The next deadline for candidates to submit financial statements is Monday.

As of July, when federal campaign reports were last filed, Baldwin led Thompson by a clear margin - $7.1 million compared with $2.5 million. Thompson recently told the Journal Sentinel that he has raised $2 million since the primary.

"I think that Baldwin and Thompson will spend considerable sums of money - it's not like they are going to be spectators in their own race," said Mike McCabe, executive director of the Madison-based Wisconsin Democracy Campaign.

"But it sure looks like the interest groups are doing a great deal of the talking."

The Center for Responsive Politics, using Federal Election Commission data, analyzes outside spending and updates figures on a nearly daily basis, since groups must disclose spending within 24 or 48 hours, depending on the circumstances.

On Friday, the center's figures show that Crossroads GPS has spent the most of any group - $3.4 million against Baldwin. Crossroads GPS and its sister group, American Crossroads, were co-founded by Rove, a Republican political consultant and former senior adviser to President George W. Bush.

Next highest: the Democratic Senatorial Campaign Committee, which has spent $3 million, and Majority PAC, which has spent $2 million - both of them against Thompson. The groups were both formed to support Democrats running for the Senate.

Americans for Prosperity, a group backed by the Koch brothers, has also put $2 million into the race. Koch Industries Inc. has interests in Wisconsin, including a Georgia-Pacific plant in Green Bay. The group is opposed to Baldwin.

All four groups are spending millions in other states, as well.

To date, nearly $9 million has been spent to oppose Thompson and $5.4 million against Baldwin, according to the center's analysis.

"One of our missions is to win a Republican majority in the U.S. Senate, and we think that Wisconsin very much helps achieve that end," said Nate Hodson, a spokesman for American Crossroads.

"Beyond that, Tammy Baldwin has been ranked as one of the most far-left members of the House, and you would expect the same if she went to the Senate," he said.

Matt Canter, spokesman for the Democratic Senatorial Campaign Committee, said: "This is a race that very well could decide whether (Republican Kentucky Sen.) Mitch McConnell is majority leader, and that's something that we don't think the majority of Wisconsin voters want.

"We are 110% behind Tammy, and we are going to make sure that she has resources she needs to win."

Outside spending is used for everything from website design and travel expenses to direct mail brochures and salaries and benefits campaign workers trying to get out the vote.

In Wisconsin, the smallest individual expense: 37 cents for a phone call from the National Rifle Association's political action committee in support of Thompson, according to federal election records.

More typical, however, is the millions spend by outside groups on 30-second TV spots, which have usually been the most negative in a race marked by attack ads.

Outside money has been used to produce spots that have labeled Baldwin's voting record as "extreme." They've also depicted her, in an unflattering way, angrily yelling: "You're damn right we're making a difference."

Third-party groups have used ads to claim Thompson cashed in on his Washington connections after leaving his post as health secretary under Bush. One ad also has Thompson saying, "Who better than me . . . to do away with Medicaid and Medicare?"

The surge in spending by third-party groups has been driven by a trend where financial clout is moving away from the traditional parties. More recently, the influence of those groups has been attributed to changes in election laws.

"The money is easier to raise because of Citizens United, so more groups have formed to take advantage of it," Duffy said.

She was referring to the U.S. Supreme Court decision that conferred most free-speech rights to corporations as individuals.

While major corporations haven't showered elections with money as some had expected, political committees known as Super PACs and other groups have attracted huge sums from liberal and conservative backers alike. In certain cases, the source of the funding doesn't need to be disclosed.

Advertising by third-party groups has influenced the Wisconsin race.

Duffy said that the Club for Growth, an antitax group that supported Mark Neumann in the Republican primary, hammered Thompson with more than $1 million in ad buys and forced Thompson to spend heavily before getting to the general election.

"Thompson really needed time to regroup afterward, pivoting to the general election, but he also had to raise money," Duffy said. "Baldwin used that time very effectively. She got two or three weeks of purely positive ads. I think that was very helpful to her."

Thompson has narrowed Baldwin's advantage in the polls, coinciding with an influx of spots by his own campaign and from the independents that paint Baldwin as out of step with the state.

But Larry Sabato, a political scientist at the University of Virginia, wonders whether the spending has reached a point of diminishing returns. Virginia is being flooded with ads from outside groups, too.

"Politics is my field and I love it," Sabato said.

"But I watch no television without my thumb on the mute button, ready to stop all advertising at each break.

"Now, if I'm not watching, I know most people aren't - they just can't take it anymore."