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A massive infusion of outside money has made the race between Republican Tommy Thompson and Democrat Tammy Baldwin the most expensive U.S. Senate race in Wisconsin history.

The price tag: $40 million in spending, according to the latest figures.

The cost of the election is sure to rise even higher between now and Nov. 6.

The largest share of the money has come from independent groups - both liberal and conservative, with most of the dollars used to oppose a candidate. So far, outside groups have spent $27.7 million in Wisconsin, according to the Federal Election Commission. That's second only to $31.7 million in another close Senate race in Virginia.

In Wisconsin, the figures include $11.6 million to oppose Thompson and $9.2 million to oppose Baldwin, according to the Center for Responsive Politics.

However, anti-Baldwin spending has jumped recently at a time when her recent lead over Thompson has evaporated, according to independent polls.

In a little more than a week, spending against Baldwin has climbed nearly $4 million, analysis by the Center for Responsive Politics shows.

The two biggest expenditures in the Wisconsin Senate race are $4.6 million from Crossroads GPS, a group founded by GOP Strategist Karl Rove, and $4.4 million from the Democratic Senatorial Campaign Committee, a pro-Democratic group.

The influx of money is a clear sign of the importance of the Thompson-Baldwin matchup, with the flood of money paying for TV attack ads that often paint Baldwin as an extreme liberal who is out of step with the average voter and Thompson as a Washington insider intent on gutting entitlement programs like Medicare.

As of Sept. 30, Baldwin's campaign had spent $8.5 million and the Thompson camp had spent $3.8 million, the Federal Election Commission records show.

By comparison, overall spending in 2010 in the race between Republican Ron Johnson and Democrat Russ Feingold cost $33.8 million, with businessman Johnson kicking in $8.9 million of his own money and only $3 million coming from outside groups.

Add Feingold's spending over the six-year election cycle and the cost of the 2010 campaign jumps to $38.3 million, election figures show.

In older elections, Feingold's race in 2004 against Republican Tim Michels cost just under $15 million over a two-year period.

The 1988 election between Democrat Herb Kohl and Republican Susan Engeleiter cost slightly more than $10 million over a two-year period.

In this year's Senate race, Baldwin's campaign had raised $11.3 million compared with $5.7 million for Thompson as of Sept. 30.

The big difference this year is the growth in money going to political action committees, known as Super PACs, and other organizations that buy ads and flood homes with direct mail and phone calls independently of the campaigns.

Heavy hitters in the political finance world dominate the donations to those organizations. In the Wisconsin race, Baldwin has been targeted by groups that received money from billionaires David and Charles Koch, and Thompson has been targeted by groups that received money from billionaire George Soros and James H. Simons, a New York hedge fund manager.

"Clearly, the independent groups have opened a new pipeline of money that didn't exist a few years ago," said Mike McCabe, executive director of the Wisconsin Democracy Campaign, a campaign finance watchdog group.

After the release of the Marquette University Law School poll Wednesday, pollster Charles Franklin said advertising, especially the money from outside groups, was a key reason for the race being so close.

The poll showed Thompson leading 46% to 45%, a virtual tie.

"At this point the ads have had their effect," Franklin said.