Thompson is blaming the slip, in part, on the rocky campaign of Republican presidential nominee Mitt Romney.
"The presidential thing is bound to have an impact on every election. You know, whether you're a Democrat or Republican. If you're a standard-bearer for the presidency is not doing well, it's going to reflect on the down ballot," Thompson said.
The new poll -- out Wednesday, September 19th from Marquette University Law School -- showed Thompson trailing Baldwin by nine points among likely voters in the Senate race, viewed as one of the most competitive in the country.
That's a major swing from one month ago, when the same poll showed Thompson leading Baldwin by nine points.
Other recent polls have shown a tighter contest in Wisconsin, in both the presidential and Senate races.
A CBS/New York Times/Quinnipiac survey of likely Wisconsin voters released on Wednesday, for instance, showed Obama leading Romney by six points, while Thompson and Baldwin were tied.
Thompson also blamed the bad poll number on the fact that he had to "regroup" after a difficult and expensive GOP primary fight. Baldwin did not have a primary.