Four Senate primaries to watch Tuesday

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(CBS News) While all eyes this week are on Mitt Romney and his newly-selected running mate Paul Ryan, four states on Tuesday are holding Senate primaries that merit some attention. Here's a look at today's races:

Wisconsin: In Ryan's home state, four Republicans are competing in today's primary to run against Democratic Rep. Tammy Baldwin for the seat that retiring Democratic Sen. Herb Kohl is leaving.

Republican Tommy Thompson, the former governor and cabinet official under President George W. Bush, is one of the best-known politicians in Wisconsin, but he's in a tight race.

The latest Marquette University Law School poll shows Thompson leading with 28 percent, but businessman Eric Hovde is trailing closely with 20 percent, as is former Rep. Mark Neumann with 18 percent. Assembly Speaker Jeff Fitzgerald took 13 percent.

No Republican has held the Senate seat since 1957, the Associated Press reports. The state, however, has been uniquely politically volatile in the past few years, and Republicans see an opening.

Connecticut: Republican Senate candidate Linda McMahon this year hasn't yet dropped the nearly $50 million she spent on her failed 2010 Senate race, but she is nevertheless outspending her primary opponent, former Rep. Chris Shays. McMahon, the former World Wrestling Entertainment CEO, appears poised to defeat the more moderate Shays.

On the Democratic side, Rep. Chris Murphy is leading in the primary against former Secretary of State Susan Bysiewicz. If he were to win, Murphy would be the favorite going into the race to replace retiring Sen. Joe Lieberman.

Minnesota: First-term Democratic Sen. Amy Klobuchar is expected to defeat three primary challengers Tuesday night. On the Republican side, state Rep. Kurt Bills has the endorsement of the Republican party and is expected to defeat two other primary contenders.

Florida: After watching a handful of other candidates drop out of the Republican primary, Rep. Connie Mack is poised to defeat former Rep. Dave Weldon on Tuesday.

Mack will take on incumbent Democratic Sen. Bill Nelson, who faces a minor primary challenger.