The contest for the Democratic gubernatorial nomination that pits Delaware's lieutenant governor against the chief executive of the state's most populous county is the marquee race in Tuesday's primary election.

Democrats also are voting in a U.S. House race where the favored candidate if elected would be the first openly transgender person in Congress. That would join another trailblazing race in November in which the lone Democratic candidate for an open U.S. Senate seat could become one of only two Black women in the Senate next year.

Meanwhile, Democratic Gov. John Carney is hoping to continue his long career in politics by winning his party's nomination for mayor of Wilmington.

Polls are open from 7 a.m. to 8 p.m.

Here's a closer look at those key races:

DEMOCRATIC PRIMARY FOR U.S. HOUSE

State Sen. Sarah McBride would move one step closer to becoming the first openly transgender person elected to the U.S. Congress with a win in the Democrat primary on Tuesday. Delaware's lone U.S. House seat is being vacated by Rep. Lisa Blunt Rochester, who has no primary opponent as she seeks the U.S. Senate seat being vacated by Democratic Sen. Tom Carper, who held the seat since 2001.

McBride faces only token opposition in the primary from businessmen Earl Cooper and Elias Weir, neither of whom reported raising any money for their campaigns. Cooper is a political newcomer, while Weir finished dead last in a 2016 congressional primary with less than 1% of the vote. McBride, meanwhile, has raised almost $3 million in contributions from around the country.

McBride achieved national recognition at the 2016 Democratic National Convention as the first openly transgender person to address a major party convention in the United States.

The winner of Tuesday's primary will go up against either Donyale Hall, a Dover businesswoman and a Gulf War-era veteran of the U.S. Air Force, or James Whalen IIII, a retired state police officer and construction company owner from Millsboro, who are facing off in the GOP primary. Democrats have held the seat since 2010.

Meanwhile, with a victory in November Blunt Rochester could become one of only two Black women in the Senate next year, joining Angela Alsobrooks of neighboring Maryland if she is also victorious in her campaign.

DEMOCRATIC GUBERNATORIAL PRIMARY

Lt. Gov. Bethany Hall-Long, who has held public office since winning a state House seat in 2002, is hoping to overcome a campaign finance scandal and succeed Carney, who can't run for governor again due to term limits. Hall-Long has been endorsed by Carney and Delaware's Democrat Party establishment.

But the two-term lieutenant governor is facing a tough primary challenge from New Castle County Executive Matt Meyer, who has raised substantially more money and has repeatedly drawn attention to Hall-Long's campaign finance violations. Former state Environmental Secretary Collin O'Mara also is seeking the Democratic nomination, but has been overshadowed by the other two candidates.

Hall-Long's campaign finance scandal surfaced last September, when she abruptly postponed a campaign event with Carney, saying she needed to attend to "a personal, private matter."

In reality, her campaign was in disarray after people hired to lead it discovered major discrepancies while reviewing years of finance reports. The scandal led to a series of resignations among top campaign staff and prompted election officials to commission a forensic audit. The audit found that Hall-Long and her husband had received payments totaling $33,000 more than what she purportedly loaned to her campaign over several years -- with many of the loans never being reported in the first instance.

It also found that Hall-Long's husband and former campaign treasurer, Dana Long, wrote 112 checks to himself or to cash. The checks totaled just under $300,000 and should have been reported as expenditures. Instead, 109 were never reported in initial finance reports, and the other four, payable to Dana Long, were reported as being made to someone else.

Despite the violations, Delaware's attorney general and elections commissioner, both fellow Democrats, declined to seek criminal charges against Hall-Long.

DEMOCRATIC PRIMARY FOR WILMINGTON MAYOR

Carney is prohibited by law from seeking a third term as governor, but he wants to remain in public office as a chief executive and is seeking the Democratic nomination for mayor of Wilmington, Delaware's largest city.

His opponent is Velda Jones-Potter, a former Wilmington city treasurer who lost a bid for mayor four years ago. Potter served a two-year stint as Delaware's state treasurer after being appointed to that post in 2008, but she lost an election for a four-year term as treasurer in 2010.

The winner of Tuesday's primary will face no opposition in November. Carney has said as mayor he would build on the investments his gubernatorial administration has made in Wilmington, with a focus on improving public schools, expanding affordable housing and helping small businesses.