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In a major turnabout, Democrat Tammy Baldwin surged ahead in a Marquette University Law School poll Wednesday in the race for U.S. Senate, taking a 9-point lead over Republican Tommy G. Thompson.

In another poll released on Wednesday by Quinnipiac University, The New York Times and CBS, Baldwin and Thompson were tied. Those results represented a drop in support for Thompson.

The latest Marquette poll shows Baldwin with a 50% to 41% lead over Thompson among likely Wisconsin voters.

The results are the exact opposite from the last Marquette poll on Aug. 22 when Thompson held a 50% to 41% advantage.

Marquette's pollster Charles Franklin described the latest findings as a "big Baldwin pickup."

"It's quite dramatic how much the shift has been," he said.

But he cautioned that the poll is a snapshot, and he said that future advertising, the presidential election and other factors are likely to change the picture before the Nov. 6 election.

The poll of 601 likely voters took place Sept. 13-16. Marquette's polling uses both cell and landline phones and has a margin of error of plus or minus 4.1 percentage points.

The winner will take the seat of retiring Sen. Herb Kohl, a Democrat.

The biggest change from the previous Marquette poll came from independents.

In August, independents preferred Thompson 47% to 37% among likely voters. The tide has turned, with independents favoring Baldwin 50% to 38% in the latest poll.

Baldwin was also able to solidify her support among Democrats. It rose from 84% to 90%, while Thompson had the same support among Republicans as a month ago - about 93%.

Franklin said that Baldwin, a congresswoman from Dane County, benefited from a post-Democratic convention bump.

President Barack Obama's support also improved in Wisconsin. He took a 54%-40% lead over Republican Mitt Romney in the latest Marquette poll. Obama had a 3-point lead over Romney in August.

Another important factor: Baldwin has been helped by heavy advertising from her own campaign and independent groups. Thompson has been less visible on the airwaves.

In a sign of the national importance of the Wisconsin seat, outside groups have spent millions of dollars in independent expenditures in the race.

A total of $8.8 million in independent dollars has been spent so far, according to the Center for Responsive Politics. That ranks second among U.S. Senate races across the country, the group said.

Among the spending in support of Baldwin has been $1.3 million by Majority PAC, a group founded by Democratic strategists, and $1.1 million from EMILY's List, a group that supports Democratic female candidates who support abortion rights, the center said.

Slightly more than $4 million has been spent to oppose Thompson, including money from groups that supported other Republicans in the primary, the center's figures show.

Thompson got a boost this week, however. On Tuesday, Crossroads Grassroots Policy Strategies, a group founded by GOP strategist Karl Rove, launched a $961,000 ad buy in Wisconsin that depicts an angry Baldwin and says she supports "bigger government, extreme politics."

With no primary, Baldwin heavily outspent Thompson through July, according to the Federal Election Commission. Baldwin spent $4.7 million and Thompson doled out $2.1 million in his primary win over three Republicans.

In the Quinnipiac/New York Times/CBS poll, Thompson and Baldwin were tied at 47% each.

The latest Quinnipiac poll represents a slide for Thompson, who led 50%-44% in the last poll on Aug. 23. Thompson and Baldwin were tied in an Aug. 8 poll at 47%.

Two other polls released this week also indicated Baldwin was leading the race.

In a statement, Baldwin's spokesman John Kraus said: "Four polls released this week show that we are moving in the right direction as the choice for Wisconsin voters has been made clear."

Thompson's spokeswoman Lisa Boothe said in a statement that Marquette's poll oversampled for Democrats, which skewed the results in Baldwin's favor.

"We do not believe that today's Marquette poll reflects the opinions of Wisconsin voters," Boothe said.

But on Tuesday at a Milwaukee manufacturing plant, the former governor had expressed confidence in the accuracy of Marquette's survey work.

"Let's see what the Marquette poll says tomorrow," he said. "I feel very good about where I am at - feel very comfortable about the future of this race."

At the factory, Thompson also acknowledged that heavy spending by Baldwin and outside groups would have an effect on latest poll results. Since the primary, Thompson has said he's had to spend lots of time raising money and limit his ad buys.

Since the primary, Baldwin has aired three ads and Thompson has aired two.

Franklin, a political scientist who is on leave this year from the University of Wisconsin-Madison, stood by Marquette's polling techniques.

If the results were adjusted to the average participation for Republicans and Democrats this year, Baldwin would lead by 48%-43%, Franklin said.

"With the adjustment, it doesn't flip who's ahead," Franklin said.