Voters head to the polls today in North Carolina to cast ballots in a primary election with major implications for the fall midterms.

North Carolina Republican senatorial candidates, from left, Mark Harris, Heather Grant, Greg Brannon, and Thom Tillis, applaud after a debate at Davidson College in Davidson, N.C., Tuesday, April 22, 2014. (AP Photo/Charlotte Observer, Todd Sumlin, Pool)

The marquee race is the Republican primary for U.S. Senate. There are also a handful of U.S. House primaries worth watching. To top it off, there is an added layer of intrigue we don't see in most states: the possibility of a runoff.

We know that's a lot. But don't worry! Below we give you everything you need to know about the primary. For more, stay tuned to Post Politics and The Fix this evening where you'll find news and analysis of the results.

You said the Senate primary is the headliner. Why?

North Carolina is one of the most important races in the battle for the Senate. Polls show Sen. Kay Hagan (D) is vulnerable. Her likely Republican opponent is state House Speaker Thom Tillis (R). The GOP establishment believes Tillis is the most electable challenger but he's not the only one running for the GOP nomination. There are seven other Republicans in the mix including tea party-aligned obstetrician Greg Brannon and tea party-aligned pastor Mark Harris. The big question is not whether Tillis will receive the most votes; that's pretty much a done deal. It's whether he will have to endure a runoff.

Runoffs? You kiddin' me? Runoffs? What's the deal with that?

Nice Jim Mora reference! Under North Carolina law, if the winner of the primary does not receive more than 40 percent of the vote, the second place finisher can ask for a July 15 runoff against the leading vote-getter. Tillis has been polling at right around 40 percent. While there is little doubt that Tillis will eventually be the GOP nominee, if he has to fight two more months for the nod, that's two months -- not to mention substantial heaps of cash -- not spent running against Hagan. That would be bad news for Republicans.

Why does North Carolina have this runoff law anyway?

It's mostly Southern states that have runoff systems. University of Georgia political scientist Charles Bullock explained it this way to the Atlanta Journal-Constitution in 2009:

Bullock said Georgia and most other Southern states implemented a runoff system in the early 20th century as an attempt to broaden the base of the then-dominant Democratic Party and finish off an ailing Republican Party. At first, he said, the runoff system was dictated by the Democratic Party executive committees at the county level. It was codified into state law in the 1960s.

More commonly, states with runoff systems require the leading vote-getter to obtain a majority of the vote to clinch a win. But North Carolina lowered its threshold to 40 percent in 1989.

Only once in the last 80+ years has the leading vote-getter in the GOP Senate primary won less than 40 percent in North Carolina, according to the University of Minnesota's "Smart Politics."

What other races are worth watching?

Rep. Walter Jones (R), known for bucking his party, is the U.S. House incumbent most at risk of losing in a primary. He faces former George W. Bush administration official Taylor Griffin. Keep an eye on this race.

The Democratic primary in North Carolina's 2nd district is also worth keeping an eye on. Former American Idol star Clay Aiken attracted widespread attention when he jumped in the race. But he has been outspent by former state Commerce Secretary Keith Crisco. The winner is likely to face Rep. Renee Ellmers (R) in a GOP-leaning area. Ellmers is favored in her primary.

Finally, peek in on the 7th district Republican primary. This is the seat of retiring Rep. Mike McIntyre (D) and a near sure-thing pickup for the GOP. The GOP establishment candidate is David Rouzer, who narrowly lost to McIntyre in 2012. The Chamber of Commerce released an ad last week hitting his opponent, attorney Woody White.

What times do poll close?

Polls close at 7:30 p.m. local time. They opened at 6:30 a.m.

Are any other states holding congressional primaries today?

Indiana and Ohio. House Speaker John A. Boehner (R-Ohio) is not expected to lose his primary, but it's worth noting that he did go on TV, a sign that he did not take the race for granted. Rep. David Joyce (R), meanwhile, faces a challenger running to his right, but is favored to advance. In Indiana, no U.S. House incumbents appear to be at serious risk of losing in the primary.