Voters head to the polls in 7 states: What to watch

Two marquee contests feature Trump-endorsed Republican incumbents facing off against serious challengers, testing the power of the President's nod.

On the left, several senior Democratic representatives in New York are facing younger, more liberal challengers who could upend the party's House leadership. Out West, a familiar face should emerge as the likely next senator from Utah.

Here's what you need to know

Polls close at 7 p.m. EDT in South Carolina. Gov. Henry McMaster was forced into a surprise runoff against Marine veteran John Warren. President Donald Trump personally campaigned for McMaster in the Palmetto State on the eve of the election, demonstrating the race's importance for the White House.

Polls close at 8 p.m. EDT in Maryland, Mississippi and Oklahoma. Republicans in Mississippi's 3rd District vote in a runoff for retiring Rep. Gregg Harper's seat. While Maryland has few competitive general election matchups, the Democratic gubernatorial primary has drawn national attention and endorsements from potential 2020 Democratic presidential candidates. Oklahoma's Republican primary for governor has 10 candidates, and will almost certainly go to an August 28 runoff, unless someone tops 50%.

Polls close at 9 p.m. EDT in Colorado and New York. Colorado's independent voters can choose to participate in either the Democratic or Republican primaries and there are competitive fields on both sides of the gubernatorial contest. A number of New York City Democrats face liberal challengers, and the Staten Island-based 11th District features a compelling contest between Trump-endorsed incumbent Rep. Dan Donovan and former Rep. Michael Grimm. Polls close at 10 p.m. EDT in Utah. The main event in the Beehive State is the Republican Senate primary, where Mitt Romney is heavily favored to win the nomination and the general election in the fall.

Here's a closer look at what to watch tonight

THE TRUMP BUMP: Can Trump bring his favorite candidates over the finish line? We're about to find out. In New York's 11th District, incumbent Rep. Dan Donovan, who Trump endorsed in a tweet, is squaring off against former Rep. Michael Grimm, who resigned from Congress in 2015 after pleading guilty to felony tax evasion, for which he spent seven months in jail.

Although Trump endorsed establishment-favorite Donovan, Grimm has tied himself to the President in the Staten Island district. The winner will face probable Democratic nominee Max Rose, an Army veteran. The race is rated Lean Republican by CNN, but, as Trump indicated in a tweet, the prospect of a Grimm vs. Rose race makes Republicans nervous.

Trump's strength will also be tested down South in the Republican gubernatorial runoff in South Carolina, where he has campaigned for incumbent Gov. Henry McMaster. McMaster, who was one of Trump's earliest backers in 2016, became governor after Nikki Haley was confirmed as the United States ambassador to the United Nations in 2017. McMaster failed to secure 50% of the vote in a crowded June 12 primary and faces former Marine and Greenville businessman John Warren on the ballot. The victor will compete against Democrat James Smith, an Afghanistan War veteran who was awarded a Purple Heart.

ROMNEY RETURNS: Longtime Republican Sen. Orrin Hatch of Utah announced he would retire at the beginning of 2018, paving the way for the return of 2012 Republican presidential nominee Mitt Romney to the national political stage. Romney announced his candidacy for the seat in February and immediately became the front-runner to become Utah's next senator. Romney failed to secure the GOP nomination outright at the Utah's Republican Party convention in April, where delegates voted for state Rep. Mike Kennedy over Romney by 51-49% (either needed 60% to avoid a primary). Nevertheless, Romney remains the heavy favorite to win the primary against Kennedy and the general election in the fall. As a critic of some of Trump's policies, he could fill a void being left by outgoing Sen. Jeff Flake of Arizona as a thorn in Trump's side in the Senate. Romney wrote in a recent op-ed for The Salt Lake Tribune that "I have and will continue to speak out when the president says or does something which is divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions."

CHALLENGES FROM THE LEFT: Three high-profile New York City Democrats are fending off millennial primary challengers from their left flanks. In the Brooklyn-area 9th District, six-term incumbent Rep. Yvette Clarke is contending with the 30-year-old Adem Bunkeddeko, a graduate of Harvard Business School and the son of Ugandan refugees. Bunkeddeko faces an uphill battle but was endorsed by The New York Times.

In the 12th District, which spans much of the East Side of Manhattan as well as parts of Brooklyn and Queens, 13-term incumbent Rep. Carolyn Maloney will face hotel executive Suraj Patel. The 34-year-old Patel has proved a formidable fundraiser, amassing over $1.2 million, almost matching Maloney's $1.6 million haul. Maloney is no stranger to well-funded opponents; in 2010, she defeated lawyer Reshma Saujani, who raised $1.5 million, by 81%-19%.

In the Queens-based 14th District, powerful 10-term incumbent Joe Crowley, who is often mentioned as a possible successor to California's Rep. Nancy Pelosi as Democratic leader (perhaps as early as next year), is being challenged by activist Alexandria Ocasio-Cortez. Though Ocasio-Cortez has been outraised by Crowley 10-to-1, the 28-year-old has captured the attention of progressive groups across the country.

ROCKY MOUNTAIN RACES: The race to replace outgoing Democratic Colorado Gov. John Hickenlooper is wide open on both sides and should be one of the most competitive gubernatorial contests of the cycle. Boulder-based Rep. Jared Polis is running against three opponents on the Democratic side, including Colorado Lt. Gov. Donna Lynne, former state Treasurer Cary Kennedy and former state Sen. Mike Johnston. Polis, who would be America's first openly gay elected governor, has run on a very liberal platform that includes single-payer health care and universal preschool. Republicans also have a four-person field, with current state Treasurer Walker Stapleton taking on former state Rep. Victor Mitchell, investment banker Doug Robinson and former Parker Mayor Greg Lopez. The Republican candidates have embraced the Trump agenda on most issues except for trade and tariffs.

One of the Democrats' top House-flip targets is also in Colorado: Republican Rep. Mike Coffman in the 6th District. This suburban Denver district is increasingly Hispanic and voted for Hillary Clinton by 9 points in 2016. Coffman's likely Democratic opponent is Jason Crow, an Army veteran who has put up strong fundraising numbers and could give Coffman -- who fielded well-funded Democratic opponents in 2016 and 2014 -- his most serious challenge yet. CNN rates the race as a toss-up.

MARYLAND DEMOCRATIC GOVERNOR: In Maryland, a large field of Democrats are clamoring for the chance to challenge one of the most popular governors in the country, Republican Larry Hogan. Leading the pack are Prince George's County Executive Rushern Baker III and former NAACP President Ben Jealous, though polls continue to show high numbers of undecided voters. Jealous has attracted support from several Democratic possible 2020 presidential contenders, including Sens. Bernie Sanders of Vermont, Kamala Harris of California and Cory Booker of New Jersey, as well as Los Angeles Mayor Eric Garcetti. Baker has been endorsed by former Maryland Govs. Martin O'Malley and Parris Glendening, as well as Sen. Chris Van Hollen, Maryland Attorney General Brian Frosh and the powerful president of the Maryland state Senate, Sen. Mike Miller Jr. Whoever emerges will be an underdog heading into the general election: Despite a friendly environment for Democrats nationwide, Hogan maintains sky-high approval ratings in the state.