Voters in six states - Colorado, Maryland, New York, Oklahoma, South Carolina and Utah - head to the polls Tuesday to pick nominees in critical races ahead of November's midterm elections.

Leading contenders include two African-American candidates fighting to become Maryland's next governor, the first openly gay man with a strong shot at a governorship and a former presidential nominee who's likely to claim a seat in the U.S. Senate.

Here are the key races to watch as results roll in:

Romney will finally beat Kennedy

Twenty-four years ago, Mitt Romney began his political career by mounting a surprisingly strong challenge to then-Sen. Ted Kennedy (D-Mass.). Romney came up short, but his first race put him on a path that led to the Republican presidential nomination in 2012.

Now, running to replace retiring Sen. Orrin Hatch (R-Utah), Romney is likely to exact a small measure of revenge against a Kennedy - though not one who's related to the Massachusetts clan. A poll conducted last week showed Romney leading state Rep. Mike Kennedy (R) 65 percent to 23 percent.

Romney is all but certain to skate to the Senate in November. He will face Salt Lake County Councilwoman Jenny Wilson (D). Utah has not elected a Democrat to the Senate since Frank Moss won reelection in 1970. Hatch beat Moss six years later.

Big races in the Big Apple

New York voters will pick party nominees in federal races Tuesday, though they have to wait until September to nominate candidates for statewide office.

Four incumbents face credible threats in their home districts, led by Rep. Dan Donovan (R), whose predecessor, ex-Rep. Michael Grimm (R), is mounting a comeback bid. Democratic Reps. Carolyn Maloney, Joseph Crowley and Eliot Engel are under pressure too, though all three remain the heavy favorites in their New York City-based seats.

Democrats see opportunities this year to capture at least four upstate districts held by Republicans, especially if a blue wave begins to develop. The party faces competitive primaries in districts held by GOP Reps. John Faso, John Katko and Elise Stefanik. Democrats rallied around state Assemblyman Anthony Brindisi (D), who's mounting a bid against Rep. Claudia Tenney (R).

And observers expect state Assembly Majority Leader Joseph Morelle (D) to claim the Democratic nomination in New York's 25th District, left vacant by the death of Rep. Louise Slaughter (D) earlier this year.

Future of Democratic Party at stake in Maryland

Only two African-Americans have won governorships in U.S. history. On Tuesday, Maryland voters will face an almost unprecedented showdown between two prominent African-American candidates running for the state's Democratic gubernatorial nomination.

The front-runners are Rushern Baker, the Prince George's County executive, and Ben Jealous, the former head of the NAACP. A third leading contender, Baltimore County Executive Kevin Kamenetz, died suddenly last month.

Baker is seen as the more pragmatic candidate, while Jealous has raced to embrace a more progressive agenda. The results will hint at which direction voters in this deeply blue state want their party to take. Baker has support from local Democratic stalwarts like Sen. Chris Van Hollen and former Gov. Martin O'Malley, while Jealous has backing from Sens. Bernie Sanders (I-Vt.) and Kamala Harris (D-Calif.).

Polls show a tight race, with Jealous on the rise. But Baker has a firm base in the vote-rich Washington suburbs; four years ago, when then-Lt. Gov. Anthony Brown (D) won the Democratic primary, he took 50,000 more votes out of Prince George's County than his two leading rivals combined.

The winner faces a tough task in November against popular Gov. Larry Hogan (R), whose approval rating even among Democrats is north of 50 percent. But in a blue wave, the Democratic nomination is worth having: Maryland's last Republican governor, Bob Ehrlich, lost his reelection bid to O'Malley even though his approval rating was strong.

McMaster in command

A majority of South Carolina Republicans voted against Gov. Henry McMaster (R) in this month's gubernatorial primary, sending him to a runoff against businessman John Warren (R).

Warren has won support from the third- and fourth-place finishers in the race, and from Rep. Ralph Norman (R), who represents a district just south of the North Carolina state line. He's spent freely of his own money in hopes of scoring a big upset in his first run for office.

But McMaster will get a big boost Monday when President Trump travels to the Palmetto State to stump for the incumbent. McMaster, the first statewide official to endorse Trump during the 2016 GOP primary season, is in office because of Trump: He ascended after Trump picked his predecessor, Nikki Haley, to be U.S. ambassador to the United Nations.

Trump's endorsement hasn't always guaranteed victory - just ask former Sen. Luther Strange (R-Ala.) and Pennsylvania state Rep. Rick Saccone (R). But in South Carolina, the last-minute stop may be the edge McMaster needs to get those final few percentage points to get to 50.

The most expensive primary

David Trone made his fortune building Total Wine & More into one of the country's biggest booze purveyors. In the last four years, he's spent a ton of that fortune trying to win a seat in Congress.

Trone faces a crowded field of Democrats in the race to replace 2020 presidential candidate Rep. John Delaney (D), in a heavily gerrymandered district that stretches from the D.C. suburbs into Western Maryland.

When he ran for an open seat in a neighboring district in 2016, Trone spent more than $13 million of his own money - and finished second in the Democratic primary. This year, he's given his campaign more than $10 million. But unlike last cycle, Trone doesn't face a challenger with the grass-roots following of someone like Rep. Jamie Raskin (D), who won that pricey primary despite being grossly outspent.

Trone's most prominent challengers are state Sen. Roger Manno (D) and state Del. Aruna Miller (D). Miller is the only candidate to have raised more than $1 million. The winner of the Democratic primary is likely to keep control of a seat where Hillary Clinton beat President Trump by a 56 percent to 40 percent margin in 2016.

Picking Hickenlooper's successor in Colorado

The candidates vying to replace term-limited Colorado Gov. John Hickenlooper (D) have already spent a combined $24.6 million - more than twice what Hickenlooper and his GOP foe spent on the entire gubernatorial election four years ago.

Rep. Jared Polis (D) leads the spending race, having given his own campaign $11.2 million. He also leads former state Treasurer Cary Kennedy (D), who won the most delegates in internal party caucuses, former state Sen. Mike Johnston (D) and Lt. Gov. Donna Lynne (D).

On the GOP side, state Treasurer Walker Stapleton (R) has support from most of the state's prominent Republicans. He faces a challenge from former state Rep. Victor Mitchell (R) and Doug Robinson (R), a wealthy investment banker.

The Republican race is something of a dynastic clash. Stapleton is the second cousin of former President George W. Bush, while Robinson is Mitt Romney's nephew.

Both national Democrats and Republicans intend to take the race seriously in the fall, in a state where gubernatorial elections have been closely fought in recent years. Democrats are on a three-game winning streak right now; Republicans last elected a governor in 2002, when Bill Owens won reelection.

Oklahoma thunder

A generation of pent-up Republican ambition is unleashing itself in Oklahoma, where a half-dozen promising contenders are competing for the chance to replace term-limited Gov. Mary Fallin (R).

If no candidate reaches a majority of the vote, the top two vote-getters will head to an Aug. 28 runoff. Lt. Gov. Todd Lamb (R) and former Oklahoma City Mayor Mick Cornett (R) lead recent polls, though self-funding businessman Kevin Stitt (R) has poured big money into his own campaign.

State Auditor Gary Jones (R), former state Rep. Dan Fisher (R) and former U.S. Attorney Gary Richardson (R) are struggling to gain traction.

The ultimate winner will likely face Drew Edmondson (D), a former Oklahoma attorney general and the last Democrat to win a statewide election.

Edmondson may appear to be a long shot in a state Trump won by 36 points in 2016, but Democrats have a history of doing well in Oklahoma gubernatorial races. Fallin's predecessor, Brad Henry (D), served two terms, and Republicans haven't elected two consecutive governors since Henry Bellmon and Dewey Bartlett in the 1960s.

In the back of your mind

A busy field of Republicans is vying for the right to replace Jim Bridenstine, who left his Tulsa-based congressional seat in Oklahoma to become the administrator of NASA earlier this year. Rep. Markwayne Mullin (R-Okla.) faces three primary challengers.

Attorney Jason Crow (D) is expected to easily win his party's nomination to face Rep. Mike Coffman (R-Colo.) in the Denver suburbs. Crow's main challenger, Levi Tillemann (D), complained that the deck was stacked against him after House Minority Whip Steny Hoyer (D-Md.) asked him to leave the race earlier this year.

Two prominent Democrats are fighting for the right to replace Polis in his Boulder-based seat. Sen. Kamala Harris (D-Calif.) has endorsed former University of Colorado Regent Joe Neguse, while some progressive groups are behind former Boulder County Democratic Party chairman Mark Williams, an Air Force veteran.

Rep. Doug Lamborn (R) faces El Paso County Commissioner Darryl Glenn (R), last seen losing to Sen. Michael Bennet (D) in 2016, and state Sen. Owen Hill (R) in a Colorado Springs-based district.

And Rep. John Curtis (R-Utah), fresh off his special election win earlier this year, will face a rematch against former state Rep. Chris Herrod (R), who finished second in that special election. A Salt Lake Tribune poll of Utah Republicans found Curtis leading Herrod by a whopping 57 percent to 21 percent margin.

--This report was updated at 1:53 p.m.