2018 primary preview: Races in Kansas, Michigan, Missouri, Washington; special election in Ohio

Voters in four states – Kansas, Michigan, Missouri and Washington – will head to the polls Tuesday to determine party nominees ahead of November's midterms. There's also a special election in Ohio to fill the vacancy left by the withdrawal of Republican incumbent Rep. Pat Tiberi.

Here are three key things to watch in Tuesday's elections (follow along here for live results):

• Democrats think Danny O'Connor has a decent shot at turning a reliably Republican congressional seat in Ohio blue. President Trump rushed to the district Saturday night in an attempt to rally supporters behind Republican candidate Troy Balderson.

• If Trump-endorsed gubernatorial candidate Kris Kobach wins the Republican primary in Kansas over incumbent Jeff Colyer, it could help Democrats steal control of the statehouse in November.

¨• Incumbent Sen. Claire McCaskill (D., Mo.) is seen as vulnerable in a state Trump won by 18 percentage points in 2016. There are 11 Republicans facing off Tuesday for the chance to face her this fall.

Polls close at 7:30 p.m. Eastern time.

The most important race of the evening is a special election taking place in Ohio, where Democrats hope to steal a congressional seat Republicans have safely held for more than 20 years.

Democratic hopeful Danny O'Connor is in a dead heat with his Republican opponent, Troy Balderson, in Ohio's 12th Congressional District. In the wake of stunning defeats in both Pennsylvania and Alabama, another Republican loss in a safe district would be yet another sign that a Democratic blue wave is about to come crashing down on Washington in November.

Trump isn't taking any chances. On Saturday night, he rushed to the district he won by 11 percentage points in 2016 for a political rally in Lewis Center, where he called Balderson "really smart" and a "really hard worker." Though last week, instead of backing Balderson, Trump mistakingly tweeted his endorsement of Rep. Steve Stivers, an unopposed Republican who isn't on the ballot Tuesday night (the president later deleted that tweet).

Polls close at 8 p.m. Eastern time for most of the state., 9 p.m. Eastern time for the westernmost counties.

Trump shook up what should've been an easy primary win for incumbent Republican Gov. Jeff Colyer with his endorsement of far-right challenger Kris Kobach. If Kobach wins the nomination, it could offer Democrats the rare chance to win a statewide race in deeply conservative Kansas. State Sen. Laura Kelly is the front-runner to win the Democratic nomination against former state Rep. Josh Svaty and former Wichita mayor Carl Brewer.

A Kobach victory in Tuesday's Republican primary could also seriously impact the races for two congressional seats Democrats hope to flip in November.

In the Second Congressional District, Democrat Paul Davis faces the winner of a seven-man Republican primary that insiders reportedly fear won't yield a candidate popular enough to win in November. Davis, the former minority leader in the Kansas House, nearly defeated former Gov. Sam Brownback in 2014.

In the Third Congressional District, which Democrats held as recently as 2011, Republican Kevin Yoder will face off against the winner of a crowded Democratic field featuring six candidates. The frontrunners are 2016 nominee Jay Sidie, progressive attorney Brent Welder and Sharice Davids, who would become the first Native American woman to serve in the U.S. House if elected.

Polls close at 8 p.m. Eastern time for most of the state, 9 p.m. Eastern time for four counties in the northern peninsula.

In the Wolverine State, Democrats are eying an opportunity to flip another statehouse, as unpopular incumbent Republican Gov. Rick Snyder is retiring.

For the Democrats, former minority leader Gretchen Whitmer holds a lead in polls over opponents Shri Thanedar and Abdul El-Sayed. The winner will likely face off against Attorney General Bill Schuette, who is trying to secure the Republican nomination over Lt. Gov. Brian Calley.

Democrats also see an opportunity to flip two Congressional districts that are considered toss-ups.

In the Eighth Congressional District, Democrats are pinning their hopes on former CIA officer Elissa Slotkin, who served on the National Security Council under presidents George W. Bush and Barack Obama. If Slotkin secures the nomination, she'll face off against incumbent Republican Rep. Mike Bishop in a district Trump won by less than 7 points in 2016.

Another toss-up is the 11th Congressional District, which is a free-for-fall with a total of 10 candidates thanks to the retirement of Republican Rep. David Trott. On the Republican side, the frontrunners are former Rep. Kerry Bentivolio, state Sen. Mike Kowall and state Rep. Klint Kesto. For the Democrats, there's Sunell Gupta, the brother of CNN host Sanjay Gupta, former Obama official Haley Stevens and Fayrouz Saad, who has the endorsement of progressive Alexandria Ocasio-Cortez, an upset Democratic primary winner for Congress in New York.

Democrats are also voting to fill a vacancy in the reliably blue 13th Congressional District, where incumbent Rep. John Conyers Jr. was forced to resign over sexual harassment allegations. Six Democrats are vying for the seat, a group that includes Conyers' great-nephew, state Sen. Ian Conyers; Detroit City Council President Brenda Jones and former state Rep. Rashida Tlaib.

Polls close at 8 p.m. Eastern time.

Among the most interesting races in November will be Democratic Sen. Claire McCaskill's attempt to defend her seat in a state Trump won by more than 18 percentage points. Baring a huge upset Tuesday night in the state's Republican primary, she'll face state attorney general Josh Hawley.

The Second Congressional District is also expected to be somewhat competitive in the fall, with the winner of the Democratic primary between attorney Cort Vanostran and Army veteran Mark Osmack facing off against three-term Republican incumbent Rep. Ann Wagner.

But the most interesting election on the ballot in Missouri Tuesday night is Proposition A, a measure that would allow non-union members to opt out of paying union dues even if they benefit from collective bargaining. The result could have national implications on labor unions in the wake of an important Supreme Court ruling from June.

>> READ MORE: How the Supreme Court ruling on labor unions in Janus vs. AFSCME could upend politics in Pennsylvania

Polls close at 11 p.m. Eastern time.

Democratic incumbent Sen. Maria Cantwell, seeking her fourth term, will be the overwhelming favorite in November against the winner of a Republican scrum featuring a whopping 28 challengers. Former state Republican chair Susan Hutchinson is considered the frontrunner.

As far as congressional seats are concerned, Democrats have a good chance to flip at least two Republican-controlled districts come November.

In the Fifth Congressional District, Republican incumbent Rep. Cathy McMorris Rodgers is expected to defeat two challengers from her right — both outspoken Trump supporters. The winner will face a competitive race against Democratic nominee Lisa Brown, the former chancellor of Washington State University Spokane.

Democrats have a shot to flip the Eighth Congressional District, which is being vacated by retiring incumbent Republican Rep. Dave Reichert. Democrats have been locked in a competitive primary fight for the right to face presumptive Republican Dino Rossi. The Democratic frontrunners are all first-time candidates: pediatrician Kim Schrier, doctor Shannon Hader and attorney Jason Rittereiser.