There are primaries in four states tonight — Minnesota, Connecticut, Wisconsin and Vermont — but Wisconsin is the state to watch.
The state of play: Sen. Tammy Baldwin is one of 10 red-state Democrats defending their seats this cycle and outside GOP groups have already spent more than $12 million attacking her — before they even have a Republican nominee to challenge her.
Why it matters: This seat is crucial to Democrats maintaining some sort of balance in the Senate. But either Republican candidate is going to give Baldwin a run for her money — at least four billionaires in the state have pledged to back the Republican nominee in their fight to oust Baldwin.
- State Senator Leah Vukmir — who's got the backing of Paul Ryan and the Wisconsin GOP establishment — will face veteran and businessman Kevin Nicholson, the more outsider candidate who was previously endorsed by Steve Bannon.
Other races to watch: Democrats are fighting to take back Paul Ryan's congressional seat in Wisconsin's first district.
- Both Minnesota U.S. Senate seats are held by women: Sens. Amy Klobuchar and Tina Smith, who took over after Al Franken resigned amid sexual misconduct allegations. There could be two women fighting for this seat in November — incumbent Democratic Sen. Smith, who previously served as Lieutenant Governor, and Republican State Senator Karin Housley.
- Don't miss congressional races in Minnesota's 2nd, 3rd and 8th districts where Republicans are in danger and a Democratic-held seat is open.
- Two Republican governors are running for a third term, so watch former Minnesota Governor Tim Pawlenty and incumbent Wisconsin Governor Scott Walker tonight. Three candidates in Wisconsin's Democratic primary could make history if elected in November.