Posted by Ruth Conniff on October 18, 2012

Tammy Baldwin and Tommy Thompson face each other tonight in their second debate in Wisconsin's hot Senate race.

The debate comes the day after the latest Marquette University Law School Poll showed their race--along with the Presidential contest--in a dead heat in Wisconsin.

Two weeks ago, before the first Presidential debate, the same poll found President Obama up by 11 points, and Baldwin up by 4 points.

Since then, Wisconsin's airwaves have been jammed with ads.

Thanks to Citizens United, outside groups are putting more money and buying more air time than the candidates for the first time in a U.S. Senate race in Wisconsin--making it the second most expensive race in the country.

Karl Rove's Crossroads GPS, the top spender in the race, has put in about $5 million.

That's a big change from the beginning of the campaign, when Thompson, having spent all his money in a contentious three-way primary, was slow to respond to attacks in ads by Baldwin.

Initial polls showed Thompson winning, and national groups, including the Republican Senatorial Campaign Committee and Crossroads GPS, were putting money into other states, in their effort to retake the Senate for Republicans.

But after two September polls showed Baldwin leading Thompson by 6 to 9 points, outside money came flowing in for Thompson.

With $22.5 million in spending by outside groups, the Wisconsin Senate race is now the second most expensive in the country after Virginia, according to data collected by the Sunlight Foundation.

Even as recently as the 2010 Johnson/Feingold race, the candidates did almost all the spending—a total of $3 to $4 million, says Mike McCabe of the Wisconsin Democracy Campaign. What jumps out about this race, besides the quintupled spending, says McCabe, “is the extent to which the candidates are doing less of the talking and the interest groups are doing more of the talking.”

The Marquette poll showed that negative ads supporting Thompson and attacking Baldwin were having an impact on voters in the last two weeks. While the same 48 to 49 percent agreed with the statement that Thompson "has sold out to special interests and isn't working for you anymore" as in the poll two weeks ago, the number who think Baldwin "is one of the most liberal members of Congress and is too liberal for Wisconsin" increased from 45 to 48 percent.

The candidates' latest FEC filings this week showed that the Baldwin campaign had raised a total of about $11.7 million, almost double the $6.1 million the Thompson campaign reported.

If you liked this article by Ruth Conniff, the political editor of The Progressive, check out her story "Joe Biden's Class Act."

Follow Ruth Conniff @rconniff on Twitter