The May 17 primaries, explained

Millions will head to the polls on Tuesday in the busiest election day of the year so far. Wondering who's running or why you should care? Here's everything you need to know:

Where are primaries happening on Tuesday, May 17?

Pennsylvania, North Carolina, Kentucky, Idaho, and Oregon.

What are some notable contests in each state?

In Pennsylvania, the race to replace retiring Sen. Pat Toomey (R) has been both heated and eventful. Notable frontrunners on the GOP side include former TV host Dr. Mehmet Oz, hedge fund manager David McCormick, and conservative commentator Kathy Barnette, whose late-in-the-game polls surge has left some in her party concerned about a potential upset. Democrats' leading candidates are Rep. Conor Lamb and Lt. Gov. John Fetterman, the latter of whom announced Sunday he was resting after a stroke but is "well on [his] way to a full recovery." The Pennsylvania governor's race has also pulled some attention, as far-right front runner Doug Mastriano (R) inches closer to a possible primary win. Democrat Josh Shapiro, Pennsylvania's attorney general, is running uncontested in his primary. 

In North Carolina, pundits are keeping their eye on the GOP Senate primary between frontrunners Rep. Ted Budd, former Gov. Pat McCrory, and former Rep. Mark Walker, all of whom hope to take over for retiring Sen. Richard Burr (R). On the other side of the aisle, former state Supreme Court Chief Justice Cheri Beasley has a "clear path" to the Democratic nomination, Roll Call notes. Former President Donald Trump has thrown his weight behind 26-year-old ex-college football player Bo Hines, who's running in the state's open 13th District. Hines has faced accusations of carpetbagging, but that hasn't stopped the House Freedom Caucus from spending more than $2 million to help elect him, reports FiveThirtyEight. The contest could ultimately go to a runoff, as could the GOP primary to replace controversial young Rep. Madison Cawthorn (R), who recently angered Republican leadership by suggesting he'd been invited to orgies with Washington, D.C. elites and had witnessed lawmakers doing cocaine. And former American Idol contestant Clay Aiken is taking another crack at Congress, adds Roll Call, running as a Democrat in the state's 4th District.

Meanwhile, in Kentucky, GOP Sen. Rand Paul is running against five relatively unknown Republican challengers for a third senate term, while former state representative Charles Booker leads the pack for the Democratic nomination, The Associated Press reports. And in the state's 3rd District, state lawmakers Morgan McGarvey and Attica Scott are helming the most notable Democratic battle, a fight to replace retiring Rep. John Yarmuth (D), Kentucky's lone congressional liberal.

Up next is Oregon, where both congressional and gubernatorial seats are on the line. In the governor's race, 19 candidates are running to take over from the outgoing Kate Brown (D), though an obvious frontrunner has yet to emerge, FiveThirtyEight notes. 

And finally, in Idaho, Gov. Brad Little (R) is being challenged by Lt. Gov. Janice McGeachin, "a Trump-backed conservative who issued executive orders banning mask mandates during the height of the pandemic when Little was out of state on business," AP writes. 

Any expected outcomes?

In Pennsylvania, the unconventional Fetterman is expected to prevail over establishment pick Lamb. But the Republican senate primary is a bit more uncertain, especially as Barnette pulls off quite the last-minute surge, muses Roll Call. In the gubernatorial race, Mastriano is thought to have a slight edge in a weak, nine-candidate GOP field, per FiveThirtyEight.

In North Carolina, the Trump-backed Budd currently leads over both McCrory and Walker, in part thanks to 45's endorsement, but likely also as a result of the support of the Club for Growth, "an anti-government-spending outside group that has spent about $11 million on Budd's behalf," FiveThirtyEight writes. 

And in Kentucky, Paul, who has a "huge fundraising advantage," is "heavily favored" to win re-election come November, adds AP.

Why should I care about these races if I don't live in those states?

The midterms typically serve as a referendum on the party in power, meaning Democrats will have to work extra hard to hold onto their congressional majority this fall. Especially in a battleground state like Pennsylvania, these primaries will set the stage for that fight. "The results will tell us a lot about where the Democratic and Republican Parties are heading after the upheaval of the Trump presidency," writes the Philadelphia Inquirer. "All told," adds FiveThirtyEight, Tuesday is "the highest-stakes primary day yet."

Speaking of Trump, the 2022 primaries are also widely seen as a test of his continued hold on the GOP, both in the candidates he endorses and the platforms on which other Republicans run. Trump has endorsed several major candidates across the May 17 races, including Oz and Mastriano in Pennsylvania; Budd, Hines, and Cawthorn in North Carolina; and McGeachin in Idaho, among others.

Trump suffered his first endorsement loss of the year last week, FiveThirtyEight writes, when embattled gubernatorial candidate and Trump pick Charles Herbster failed to secure the Republican nomination for Nebraska governor. The former president did, however, see success in West Virginia with the victory of GOP Rep. Alex Mooney.