(CNN) - Five states head to the polls on Tuesday in primary elections that will set up some of the most critical contests to determine control of Congress next year.
Two of the most vulnerable senators -- a Republican in Nevada and a Democrat in North Dakota -- will have challengers by the end of the night.
In South Carolina and Virginia, Republican voters will decide if they want to punish representatives who haven't fully embraced President Donald Trump. Democrats in Nevada and Maine are picking candidates they need to turn the tide on GOP-controlled Washington.
Here's what you need to know:
Polls close at 7 p.m. EDT in Virginia and South Carolina. In Virginia, Democrats are targeting a handful of seats to flip in November and Republicans will choose their nominee to challenge Democratic Sen. Tim Kaine. In South Carolina, Rep. Mark Sanford is facing an insurgent primary challenger who is attacking him for his criticism of Trump. Polls close at 8 p.m. EDT in Maine. The state will be the first in the nation to use ranked choice in statewide elections on Tuesday. Democrats feel good about the race to replace controversial Republican Governor Paul LePage, though the state's unusual voting means even professional observers don't know which Democratic candidate will win the nomination. State Attorney General Janet Mills and Iraq and Afghanistan wars veteran Adam Cote are both seen as strong Democratic candidates for the Fall. All polls are closed at 9 p.m. EDT in North Dakota. Republicans will most likely pick Rep. Kevin Cramer, the state's sole representative in Congress, to challenge incumbent Sen. Heidi Heitkamp, one of the most vulnerable Democratic senator this November. Trump won North Dakota by 36 points in 2016. Polls close at 10 p.m. EDT in Nevada. Another competitive November election will be take shape here on Tuesday. Incumbent Sen. Dean Heller, considered to be the most vulnerable Republican senator this year, will most likely face Democratic Rep. Jacky Rosen, who is most favored to win her party's nomination in a general election race CNN currently rates as a toss-up.
Here's a closer look at what to watch tonight:
ANOTHER TRUMP CRITIC IN TROUBLE: Just one week after Republican Rep. Martha Roby was forced into a runoff in Alabama over her 2016 disavowal of Trump, South Carolina's Sanford might face a similar fate. Sanford is facing a strong primary challenge from state Sen. Katie Arrington, who has attacked him for criticizing Trump and has alluded to his adulterous past, saying in an ad that "it's time for Mark Sanford to take a hike --- for real this time." Sanford only won re-nomination by 4,000 votes in 2016, but he has also demonstrated a remarkable ability to weather political storms and controversies over the years.
WILL PARNELL WIN? Sanford is not the only politician in hot water in South Carolina. In the state's 5th Congressional District, embattled Democrat Archie Parnell -- who narrowly lost a 2017 special election -- remains on the ballot, despite recently admitting to domestic abuse in the 1970s, news which rocked his campaign and likely spoiled a long-shot Dem pick-up opportunity if Parnell advances.
MCMASTER OF HIS OWN FATE: The Palmetto State is home to one of the most compelling gubernatorial contests of the year. Republican Gov. Henry McMaster --- who endorsed then-candidate Trump in January 2016 --- earned a Trump Twitter endorsement in the final weekend heading into the primary. He faces strong challengers in Catherine Templeton and John Warren, among others. McMaster must win 50% of the vote or more in order to avoid what would be an embarrassing runoff. On the Democratic side, state Rep. James Smith -- endorsed by Joe Biden -- is seen as the one Democrat who can pull off the upset in deep red South Carolina, should a "blue wave" extend that far south.
STRONGER ALONE: After losing for Vice President in 2016, Kaine is uncontested in the Democratic primary in Virginia for his Senate re-election. The Republican primary for the seat has been another race to the right between 2017 gubernatorial candidate Corey Stewart, state Delegate Nick Freitas, and pastor E.W. Jackson, the Virginia GOP's 2013 nominee for lieutenant governor. Kaine will begin the general election as a favorite to win re-election against whichever Republican emerges.
DEMOCRATS PLAY FOR THE HOUSE: While Virginia has been trending blue at the statewide level for the past decade, Republicans have managed to have a firm grip on the commonwealth's House seats. Democrats, however, believe they can flip three, possibly four seats here come November. Their top target is Rep. Barbara Comstock's 10th District in the DC suburbs. Six Democrats are running, and whoever emerges will be in a toss-up against Comstock.
Meanwhile, GOP Reps. Scott Taylor in the 2nd District and Dave Brat in the 7th District will face credible Democrats in November. CNN rates both as Lean Republican.
GOP Rep. Tom Garrett's late retirement decision opens up his more heavily Republican 5th District, which Democrats will have more trouble putting into play. CNN rates it Likely Republican.
NEVADA HOUSE RACES: Rep. Jacky Rosen's Senate candidacy opens up her seat, where Democrat Susie Lee has emerged as the Democratic frontrunner. She should face Republican Danny Tarkanian, who lost to Rosen by under 4,000 votes in 2016. CNN rates this race as Lean Democrat. In Nevada's 4th District, Democratic Rep. Ruben Kihuen announced he would not seek re-election last year following allegations of sexual misconduct. His central Nevada district, including the Las Vegas suburbs, gives Democrats a slight advantage, as CNN rates this race lean Democrat. Former Reps. Steven Horsford (D) and Cresent Hardy (R) -- who lost to Kihuen in 2016 -- are favored to face off in November for another shot at representing this district.
MAINE VOTING ATTRACTION: Maine's contests for governor and Second House District are in Democrats' eyes to take in November, but most political watchers are looking to the Pine Tree state to see how the voting turns out. The state instituted "ranked choice voting" for elections with more than two candidates, meaning voters can rank all candidates running in either gubernatorial primary, the Democratic nomination in the 2nd Congressional District, and a GOP nod in a state legislative seat. If no one receives a majority, the last-place candidate is dropped and those voters' second choice voters get added to the original vote count. This continues until someone has a majority or the candidate with the most votes in the final two is declared the winner. The voting method is also facing a "people's veto" referendum on the ballot, meaning voters could choose to start the process to get rid of it. A "yes" vote will continue the use of ranked choice voting for primary and federal elections. A "no" vote defers implementation until 2021 and requires a constitutional amendment to be enacted to allow its use.
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