Happy primary day!

After a brief lull, primary elections are kicking back into gear Tuesday in four states. It's the start of a busy stretch of important races between now and Labor Day.

Voters in Kansas, Michigan, Missouri and Washington head to the polls. There's a lot going on.

Below we break down the four most important things you should watch. As always, stay tuned for results this evening right here on Post Politics.

1. Will Sen. Pat Roberts get past his tea party challenger?

Sen. Pat Roberts, right, walks with state Sen. Jeff Longbine through downtown Emporia, Kan., during a campaign stop Wednesday. (Dustin Michelson/Emporia Gazette via AP)

Roberts has made a couple of big mistakes, but he is favored to win. It all started with a February New York Times story revealing the senator pays rent to supporters to stay with them when he's in Kansas, instead of residing at his own house. The story fueled the kind of "he's lost touch with the state" criticism that dislodged Richard Lugar from the Senate in 2012. (Roberts owns a home suburban Washington.) Even worse, Roberts seemed oblivious to how bad it looked, jokingly telling the Times, "I have full access to the recliner." It didn't help when he remarked in July radio interview, "Every time I get an opponent — I mean, every time I get a chance, I’m home."

But here's the thing: Milton Wolf has not been able to capitalize. Wolf -- a second cousin of President Obama -- has his own problems. The doctor posted graphic X-ray images of gunshot victims on Facebook and made crude jokes about them. He has been outspent nearly 3 to 1. And while he's gotten some help from national tea party groups like the Senate Conservatives Fund, he is hardly the national tea party darling that Chris McDaniel was in Mississippi against Sen. Thad Cochran.

National Republican Senatorial Committee Chairman Jerry Moran (Kan.) didn't sound too worried about Roberts last week in a briefing with reporters. Polls have tightened a bit, but they still mostly show Roberts leading comfortably. This is one of the final chances for the tea party to knock off a sitting senator in a primary this year.

2. A tale of two Republican congressmen in Michigan 

This combo made from campaign and staff photo shows Brian Ellis, left, who is challenging Rep. Justin Amash. (AP Photo)

Reps. Justin Amash (R-Mich.) and Kerry Bentovolio (R-Mich.) have both clashed with GOP leadership. Both drew primary challengers backed by the business wing of the GOP. But Amash stands a much better chance of winning Tuesday. He's run a solid campaign, while Bentivolio -- who is one of the unlikeliest members of Congress -- has not. One poll showed Bentivlio down 22 points to businessman David Trott, who is backed by the U.S. Chamber of Commerce. Bentivolio has been badly outspent by Trott, and the congressman lost his campaign manager in the spring. Amash, whose libertarian-leaning views have irked hawkish colleagues, is leading challenger Brian Ellis, polling shows. Amash has also received reinforcements from the well-heeled Club for Growth. The GOP establishment it probably looking at a split decision in these two races.

3. Not your typical House primary in Kansas

Rep. Mike Pompeo. (Mike Hutmacher/The Wichita Eagle via AP)

When a Republican House member draws a primary challenger, that person usually runs hard to the right. That's not the case with Rep. Mike Pompeo (R-Kan.) and former congressman Todd Tiahrt. Tiahrt has defended earmarks he secured while Pompeo has railed against them. A recent poll showed Pompeo up, but Tiahrt not out of the running. Meanwhile,  keep an eye on Rep. Tim Huelskamp (R-Kan.), who is trying to fend off Alan LaPolice, a former local schools superintendent. It would be a surprise if Huelskamp loses.

4. Will the all-party primary system doom Republicans in the state of Washington?

Dan Newhouse, a U.S. House candidate running in the 4th District of Washington state speaks as other candidates listen at a June 30 forum in Othello, Wash. A total of 13 candidates are running to replace Rep. Doc Hastings (R), who is retiring after two decades in the seat. Other candidates shown here are, from left, George Cicotte, Gavin Seim, Gordon Pross, Janea Holmquist, Glenn Stockwell, Clint Didier, Richard Wright and Kevin Midbust. (Nicholas K. Geranios/AP)

California's all-party primary system nearly caused a disaster for Democrats in the 31st District this year. On Tuesday, Republicans are on the hot seat in Washington state. As in California, all candidates compete for a seat in the same primary, regardless of party affiliation. The top two finishers advance to the general election.

In the state's only seat with a chance at flipping (and it's really only an outside chance), Republicans are hoping retired Microsoft engineer Pedro Celis advances to a showdown against Rep. Suzan DelBene (D). But there are five other candidates -- including three Republicans -- in the mix.

Meanwhile, as Roll Call points out, the crowded field of Republicans and Democrats competing for the seat of retiring Rep. Doc Hastings (R) in a heavily conservative district could open the door to a vote split between the GOP candidates and potentially cause one or maybe even two Democrats advance. It's not likely, but don't rule anything out. Democrats in California's liberal-leaning 31st district were shut out of the general election in 2012, and it almost happened again this year.