Recent polls show a tight three-way race between the candidates, with Wisconsin state Assembly leader Jeff Fitzgerald (R) in a distant fourth place. The primary has taken on increasingly nasty tones, with all candidates crying foul about each other's attacks and leveling harsh charges against one another.

The winner of the Aug. 14 primary will go on to face Rep. Tammy BaldwinTammy Suzanne BaldwinAmerican women will decide who wins and loses in 2018 elections Grassley, Dems step up battle over judicial nominees 10 Senate Democrats are up for reelection in Trump country MORE (D-Wis.) in what could be a tight general election race.

The Democratic Senatorial Campaign Committee used Gingrich's endorsement to take a shot at Thompson's work for Beltway firms after he left former President George W. Bush's Cabinet.

"Newt Gingrich and Tommy Thompson lobby on behalf of special interest clients but avoid ethics laws that would require them to register as lobbyists. This baggage cost Newt the election and it is certain to cost Thompson the election as well," said DSCC spokesman Matt Canter. "Thompson's experience peddling influence might make him a great lobbyist for special interests but he'd make a terrible senator for Wisconsin families."

This post was updated at 1:55 p.m.