A quick guide to May 15 primaries in Pennsylvania, Idaho, Nebraska, and Oregon

The next big slate of primaries is up Tuesday, May 15. Voters in four states — Idaho, Nebraska, Oregon, and Pennsylvania — will head to the polls to select candidates for key congressional, Senate, and governor’s races.

Pennsylvania is the election headliner of the evening. The state is crucial for Democrats to have a shot at retaking the House of Representatives in November, and Tuesday will set up matchups for the midterms. There’s also a heated GOP gubernatorial race in Idaho, and whoever wins the Republican primary will likely become the state’s next governor.

Here’s what to watch in every state.

Polls close: 8 pm Eastern time

Pennsylvania’s primaries may be a preview of Democrats’ national prospects in November. Both a new congressional map and some open seats benefit the Democrats. Tuesday will set the general election contests and offer a clearer picture of Democrats’ chances.

Three open seats in Republican-held districts that Hillary Clinton won in 2016 — Pennsylvania’s Fifth, Sixth, and Seventh — offer solid opportunities for Democratic pickups.

There are also a few districts where Republican incumbents look shaky because they’re facing primary challengers: The 11th, a heavily Trump-voting district where Rep. Lloyd Smucker is being challenged from the right; and the 12th, where Bradford County Commissioner Doug McLinko is running against incumbent Rep. Tom Marino, who faces questions over his ties to the opioid industry. The opioid crisis is the key issue in the race.

A Senate seat is also up for grabs. Incumbent Democratic Sen. Bob Casey will await his Republican midterm challenger. Rep. Lou Barletta and state Rep. Jim Christiana are battling it out. Casey will most likely defeat either one of them in the general, but Pennsylvania, which went for Trump in 2016, can be unpredictable.

Democratic Gov. Tom Wolf is also up for reelection. He’s the favorite to win in November, but depending on who wins the GOP primary, things could get interesting. The Republican candidates are state Sen. Scott Wagner, litigation attorney Laura Ellsworth, and former paratrooper and consultant Paul Mango.

Polls close: 8 pm Mountain time or Pacific time

Idaho’s gubernatorial seat is wide open, after the Republican incumbent decided not to seek a fourth term. That led to a crowded GOP primary field, and three candidates have risen to the top: Brad Little, the current lieutenant governor and the establishment pick; Rep. Raul Labrador, a House Freedom Caucus founding member and the ultraconservative in the race; and Tommy Ahlquist, a former physician and real estate developer who’s painting himself as the outsider.

Whoever wins the primary is almost guaranteed to sail to the governorship, which is why this primary race has become so contested — and expensive.

Democrats are mostly battling it out for the honor of losing to a Republican in November. But the contest is a microcosm of the larger divisions among Democrats. Paulette Jordan, a 38-year-old two-term Idaho state legislator, has won national endorsements from progressive groups, but the state’s liberal establishment has mostly gotten behind A.J. Balukoff, an entrepreneur who launched a failed bid for governor in 2014.

Polls close: 8 pm Central time/7 pm Mountain time

Incumbent Republican Sen. Deb Fischer is the favorite to win the general. But a woman Democratic challenger, Jane Raybould, a Lincoln City Council member, could be a contender if she wins the Democratic primary. Both politicians are seen as able to work across the aisle. Fischer will still probably cruise to victory, but as Vox’s Tara Golshan writes, “if 2018 starts to get really weird, we’ll have to check back in with the Cornhuskers.”

There’s also the potential for Democrats to reclaim a House seat in Nebraska’s Second Congressional District, which includes Omaha and its suburbs. Republican Rep. Don Bacon won the seat in 2016, defeating then-incumbent Democrat Brad Ashford by a percentage point.

Ashford wants to repay the favor in 2018 and is running for the Democratic nomination. He’s the frontrunner with name recognition but faces a strong progressive challenge from Kara Eastman, a nonprofit manager.

Both Ashford and Eastman would likely be competitive in the general against Bacon. The Cook Political Report rates Nebraska’s Second a toss-up.

Polls close: Mail-in; ballots must be received by 8 pm Pacific time

Incumbent Democratic Gov. Kate Brown has the clear edge against her primary challengers. As the Oregon Statesman Journal points out, her competitors have raised no money and have no experience in elected office. Instead, it will be about the share of the vote for Brown, who won a 2016 special election but whose popularity has declined in the past year.

That may give a Republican candidate a small opening in November — but it’s still an uphill battle in deep-blue Oregon. Still, there are a few GOP candidates who are fighting it out for the nomination: state Rep. Knute Buehler, a moderate with legislative experience; former Navy pilot Greg Wooldridge, who describes himself as a “pragmatic conservative”; and entrepreneur Sam Carpenter, who is the Trump-like candidate.