The presidential race will take top billing on Tuesday, but there are several other important races worth watching, too.
Two California Democrats could advance to the general election in the state's open Senate contest. The first GOP incumbent of 2016 will lose in a bitter primary that has pitted conservatives against a former ally. And several primaries could determine how competitive House takeover opportunities are this fall.
In addition to the states voting for president, North Carolina and Iowa are also holding congressional primaries. Here's what to watch tonight:
Will two Democrats advance in the California Senate race?
Thanks to the state's "jungle primary," where the top two vote-getting candidates regardless of party advance to the general election, the race to succeed retiring Democratic Sen. Barbara Boxer could feature two Democrats.
Attorney General Kamala Harris has long led in polls, with Rep. Loretta Sanchez in second place. Both are expected to outpace a handful of GOP candidates on the ballot.
Either candidate would bring added diversity to the Senate if she wins — Harris would be just the second African-American female senator, while Sanchez would be the first Latina senator (another Hispanic woman is also running in Nevada this year).
Harris has emerged as the progressive favorite, with the backing of Massachusetts Sen. Elizabeth Warren and other liberal groups. But Sanchez is working to appeal to centrist and moderate Republican voters, which could make for a more interesting general election contest, if they both do advance.
Can conservatives win an intraparty battle in the North Carolina race?
The Tar Heel State is holding special House primaries due to mid-decade redistricting. Those new congressional lines forced two GOP incumbents into a face-off, and conservative groups are taking advantage of the intraparty fight to make an example out of Rep. Renee Ellmers.
Both fiscally conservative groups like Americans for Prosperity and the Club for Growth, along with anti-abortion groups such as the Susan B. Anthony List, are upset with votes the three-term congresswoman has made since she was first elected in 2010 with Tea Party support. They've spent more than $1.1 million opposing her re-election bid, looking to boost either fellow Rep. George Holding or physician Greg Brannon.
Ellmers already represents less than 20 percent of the new suburban Raleigh district, while the new 2nd District includes almost 60 percent of Holding's old district (even though he lives just outside its new boundaries). So while she was already at a disadvantage to begin with, the massive campaign to defeat her has made her an even bigger underdog, according to state observers.
Conservative groups hope an Ellmers loss is a warning shot to other Republicans that if they go to Washington and don't vote in line, that they, too, will face the electoral consequences. But Ellmers argues that her more pragmatic approach has enabled her to be a more effective legislator.
Democratic primaries matter for House takeover hopes
Democrats already face tough odds of being able to flip the 30 seats they need to win back the House, even with Donald Trump atop the GOP ticket. But if a handful of primary contests don't go their way Tuesday night, that climb could get even harder.
In Iowa, freshman Rep. Rod Blum is one of the most vulnerable Republicans in the country after narrowly beating former state House Speaker Pat Murphy in 2014. Murphy wants a rematch, but Monica Vernon, who became the Democrats' nominee for lieutenant governor two years ago after losing the same House primary to Murphy, stands in his way. National Democrats believe Vernon is the stronger candidate to go up against Blum, and she's proven to be the better fundraiser; if she's the nominee, the race probably tilts more in their favor.
Keep an eye on California's jungle primary to succeed retiring Democratic Rep. Lois Capps, too. There are nine candidates in the race in the Democratic-leaning Santa Barbara district, but the volatility of such an uncertain race has some Republicans believing that two Republicans could actually finish first and advance to the general election.
Prominent Democrats aren't united in who should be their standard-bearer: Capps has endorsed Santa Barbara County Supervisor Salud Carbajal in the race, while Lt. Gov. Gavin Newsom has backed Santa Barbara Mayor Helene Schneider.
That split has some Republicans optimistic that two Republicans could actually finish first and sneak into the general election, denying Democrats a seat they have to hold on to.
The Los Angeles Times also details why it's important to keep an eye on the primaries against California GOP Reps. Jeff Denham, David Valadao and Steve Knight, more seats Democrats are targeting in the fall.