GOP Senate race tops Tuesday's Wisconsin primary election.

Eric Hovde
Hedge fund manager Eric Hovde, pictured here on March 8, 2012, is one of four Republicans running in Tuesday's Senate primary in Wisconsin.
AP Photo/Scott Bauer

Voters in Minnesota head to the polls Tuesday for a primary election - and there's also a primary election in Wisconsin.

Democrat Sen. Herb Kohl is retiring this fall, leaving an open seat. The Democrats have united behind Congresswoman Tammy Baldwin, while the Republicans have four Senate candidates running in Tuesday's primary.

Shawn Johnson, the state government reporter for Wisconsin Public Radio, will join us to discuss that and other races on the primary ballot.