Democrats are defending more than two dozen Senate seats, including 10 in states President Donald Trump won in 2016, and Republicans have just eight Senate seats up for re-election.
Here's a look at what's coming up in the November midterm elections:
California
Eight of the nine House seats rated as competitive by CNN are currently held by Republicans, some of which are in districts Clinton won in 2016. If Democrats are going to take back the House, they will need (and may get) a few of these to do it. California has a competitive governor's race to replace outgoing Gov. Jerry Brown, with top contenders in Lt. Gov. Gavin Newsom and former Los Angeles Mayor Antonio Villaraigosa, both Democrats.
Iowa
Two-term GOP Rep. Rod Blum's race is rated as a toss-up. Blum won 51% and 54% in his northeastern Iowa district during his first two campaigns but could face his toughest race in a year that could favor Democrats.
Mississippi
Sen. Thad Cochran's upcoming retirement -- announced in early March -- sets up a special US Senate election in Mississippi, meaning both of the state's Senate seats will be on the ballot come November. GOP Sen. Roger Wicker is up for re-election and is facing a primary challenge from state Sen. Chris McDaniel.
Montana
Democratic Sen. Jon Tester will face voters for the third time, but this year he's coming on the heels of Trump's 21-point win in Big Sky Country. Republicans see this seat as a perfect opportunity for a flip, but party members have yet to coalesce around one candidate.
New Jersey
Five of New Jersey's 12 congressional seats are on CNN's competitive ratings this year. The two open seats left by retiring GOP Reps. Rodney Frelinghuysen and Frank LoBiondo are seen as big targets for Democrats to flip. Democratic Sen. Bob Menendez is up for a third term this year.
New Mexico
Four-term GOP Rep. Steve Pearce is running for governor, which leaves a slight opening for Democrats in this southern New Mexico district. The area trends conservative, as CNN rates this House race as likely Republican. Democratic Sen. Martin Heinrich is up for re-election.