The prelude to Wisconsin’s August 14 Primary Election keeps getting more interesting–especially in the GOP race for U.S. Senate. The upcoming retirement of Democratic Senator Herb Kohl has led Republicans hopefuls into a true donnybrook for the chance to face Democrat Tammy Baldwin in the fall.

For months, former governor and HHS Secretary Tommy Thompson was the frontrunner, but hedge fund manager and political newcomer Eric Hovde has brought deep pockets and an “outsider” brand to the race. It seems to be working, due in part to outside forces.

Thompson, the most moderate of the field, has become a punching bag for the other Republicans on the ballot, Jeff Fitzgerald and Mark Neumann. National conservative groups have embraced Neumann and have relentlessly attacked Thompson for his support of some health care reforms. The onslaught is having its effect, and Thompson has seen his lead shrink remarkably fast.

Last night, we asked 1,237 likely GOP voters in Wisconsin who they would vote for if the election were held today. Here’s what they said (results have been weighted):

Poll type:: Automated Date: July 30, 2012 - Participants: 1,237 Likely Voters - Margin of Error: ± 2.8%
Jeff Fitzgerald12.11%
Eric Hovde23.20%
Mark Neumann16.66%
Tommy Thompson22.79%

Similar to what we saw earlier this year in Nebraska’s GOP Primary, the outside forces that are pummeling away on the presumed frontrunner are allowing a relative newcomer to take a lead down the stretch. However, in Nebraska the shift in favor of Deb Fischer happened very quickly and just days before the election–leaving little time for Jon Bruning to strike back. In the Dairy State, Thompson has the time to re-group and claw his way back into the lead–especially in light of the relatively high number of undecided voters.

But he better get clawing. Right now, Hovde has the mojo.