GREEN BAY, Wis. (WFRV) - The race for Wisconsin's soon to be vacant U.S. Senate seat is tightening. A new poll released Wednesday by the Marquette University Law School shows former governor Tommy Thompson still on top with 28% support. That's just eight points ahead of businessman Eric Hovde and 10 points ahead of former congressman Mark Neumann. Assembly speaker Jeff Fitzgerald is trailing with 13 percent.

Thompson's lead has been steadily shrinking over the past couple of months. In June, he led Hovde by 20 percentage points. In July, that narrowed to 12 points. Now, just eight points separate the two.

"Probably a little surprising since I would expect from the Thompson campaign perspective, my guess is that he thought he would be able to leap into this relatively easily," Professor Scott Furlong, with the UWGB Political Science Department said.

But both Hovde and Neumann have been using TV campaign ads to boost their name recognition, try to pin themselves as the biggest conservatives in the race and show Tommy Thompson as somewhere in the middle.

"His republicanism, if you will, and he would argue against this as part of the election process, has always been seen a  little bit more moderate than a large part of the republican party," said Furlong.

The poll also shows 21% of likely voters are still undecided. Furlong believes that's due large in part to apathy.

"Another part of it is have they distinguished any differences between the candidates?" he said. "And are they ready to commit?"

With voter fatigue setting in, a low turnout is expected. Furlong said the outcome will depend on which side rallies the most support on Tuesday.

"Trying to hit it in such a way that you're not going to disadvantage yourself come the general election," he added.

Furlong doesn't believe cross-over democratic voters will make much of an impact in the primary.