A Trump or Cruz nomination would reverberate for Republicans in Senate races throughout the country. Here’s how it might play out in some key contests.
1. Ohio
In this swingiest of swing states, Rob Portman is already locked in a nip-and-tuck battle with former Democratic Gov. Ted Strickland. The incumbent is sitting on a pile of campaign cash, but it may be insufficient to protect him if a Democratic wave sweeps the Rustbelt.
2. North Carolina
Two-term incumbent Richard Burr remains the favorite, yet in this increasingly purple Southern state, an unpopular Republican at the top of the ticket could drag him under. One recent poll found voters 26% less likely to back him if he supports Trump.
3. Arizona
Onetime presidential contender John McCain should be able to hold off his credible Democratic challenger for another win in this ruby red state. Doing so will require him to keep faith with fans of Trump—who cleaned up in the primary while disparaging McCain’s war service—without alienating too many of the state’s nearly 1 million Hispanic voters.
4. Iowa
Democrats hope to nationalize the Iowa race by making Chuck Grassley, who chairs the Senate Judiciary Committee, the face of GOP opposition to allowing a vote on Supreme Court nominee Merrick Garland. Grassley remains popular at home as he seeks his seventh term, but the court fight helped recruit a tough Democratic challenger.
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5. New Hampshire
Freshman Kelly Ayotte faces a formidable task trying to win reelection in a swing state Barack Obama won twice. Trump romped in the primary, but her refusal to disavow him has become an attack line for her popular Democratic opponent.
A version of this article appears in the May 1, 2016 issue of Fortune with the headline “Down-Ballot Angst.”