In the Wisconsin Senate race, Tommy Thompson, a former Republican governor and Bush Cabinet secretary, and Democratic Rep. Tammy Baldwin are locked in a surprisingly close race.

With so much on the line, including the possibility it may determine which party holds the majority in the Senate, the tone on the campaign trail has gotten nasty.

"This is a classic race between somebody that works with both political parties and somebody like Tammy Baldwin who is so extreme that even her own political party doesn't pass her legislation," Thompson told Fox News.

Baldwin declined a request for an interview, but at a recent rally with former President Clinton, Baldwin blasted her opponent.

"What Tommy's not telling you is what he's been up to since he left the state of Wisconsin ... because, you know, he went to work in Washington for George W. Bush and helped turn Bill Clinton's surplus into a deficit," Baldwin told supporters.

Recent TV ads from the two campaigns have focused on the emotional topic of the Sept. 11, 2001, terrorist attacks.

“Tammy Baldwin had the opportunity to vote to honor the victims of 9/11, and she voted against it,” Thompson’s ad tells Wisconsin voters, referring to a 2006 resolution.

Baldwin’s campaign called the ad a disgrace and responded with one of its own.

“The truth: Time and again Tammy Baldwin has supported honoring victims of 9/11,” said Baldwin’s advertisement.

Thompson defended the message, saying it tells Wisconsin voters who she is.

"It just points out how extreme she is, out of mainstream,” Thompson told Fox News. “She's an individual who leans far left and she's a taxer and a spender, and believes that stuff."

Experts say the harsh tone is having an impact.

"Voters describe them both in terms of the negative commercials that have been aimed at them,” Charles Franklin, director of the Marquette Law School Poll, told Fox News. “And so, this is not a race to the top, it is a race to which one is less negative by Election Day."

The latest Marquette Law School Poll of likely voters has Thompson leading Baldwin by one point – 46 percent to 45 percent -- while other major polls are also within the margin of error.