Louisville, city of 600,000, has a single polling place in Kentucky primary

Overwhelmed Kentucky and New York officials faced a deluge of mail-in votes likely to delay results for days after high-profile primaries Tuesday, contests testing whether establishment Democratic congressional candidates can withstand challengers fuelled by voter fury over racism.

There was particular concern about how voting might play out in Louisville, Ky. The state's largest city and hometown of a serious challenger for the Democratic nomination for the Senate, Louisville population nearly 600,000 had just one in-person polling place, the Kentucky Exposition Center.

Each of the city's local districts was replicated inside the vast convention centre, and plenty of poll workers were available to direct voters to their designated areas to cast their ballots. And while masks weren't required, the overwhelming majority of voters appeared to wear them. Outside, buses shuttled people from huge parking lots to the centre's doors.

"They should have more [elections] like this. You can get in and get out quick," said 70-year-old Mary Moorman, an African American woman who said she did not have to wait in lines like she normally would.

But some voters said the centralized in-person polling place made voting more complicated. Michael Baker, an African American man who is also from the West End neighbourhood, was dissatisfied with the location because it was far away from where he lived.

"In my neighbourhood, most people don't have cars," he said. "It's not fair for them to have one site, and it be so far away from people who are not able to commute here."

As well, groups were forced to organize free rides for voters through buses or ridesharing companies.

The consolidation of polling places came in response to the coronavirus pandemic, which has upended life in America since March.

Like other states, Kentucky and New York have made it easier for voters to cast ballots by mail instead of risking exposure to the virus by waiting in long lines. That is likely to mean delayed election results, with far more mail-in votes than usual and ballot-counting procedures that haven't been adjusted to handle them.

Battle to take on McConnell

The focus was on two races Tuesday. In both, Democrats were waiting to see if nationwide protests sparked by last month's killing of George Floyd in Minneapolis police custody would translate to a decisive turnout by African American and progressive voters.

Amy McGrath, favoured by party leaders and buoyed by a massive $41 million US campaign fund, faced an eleventh-hour scare as she fought to become the Democratic nominee against Senate Majority Leader Mitch McConnell. She's a former Marine combat pilot with centrist views, which top Democrats like Senate Minority Leader Chuck Schumer view as a strong match for the Republican-heavy state.

But first-term state legislator Charles Booker's underfinanced campaign has caught fire after he attended recent protests against the March police killing of 26-year-old Breonna Taylor in her Louisville home. That's helped Booker win support from progressive icons like Sen. Bernie Sanders and the state's two largest newspapers, leaving the primary's outcome unpredictable.

Baker, the West End resident, decided to back Booker.

Meanwhile, Eliot Engel, embraced by a who's who of prominent Democrats, was battling for a 17th House term in New York.

The challenger to the current House foreign affairs committee chair is educator and political neophyte Jamaal Bowman, who has drawn strength from anti-racism protests and his accusations that Engel has grown aloof from his district in parts of the Bronx and Westchester.

Engel, a liberal, has support from the likes of Hillary Clinton, House Speaker Nancy Pelosi and the Congressional Black Caucus. He's also outspent Bowman, who hasbeen helped by progressive groups and a coveted endorsement by The New York Times.

Virginia was also holding congressional primaries, and there was one Republican House runoff each in North Carolina and Mississippi.

Kentucky has been overwhelmed by such an increase in mail ballots that the state's two biggest counties, Jefferson and Fayette, aren't planning to release results on election night, said Secretary of State Michael Adams. Jefferson County is home to Louisville.

Kentucky typically receives few mail ballots, but expects them to account for the majority of votes this time. With the state now allowing any registered voter to vote by mail, more than 400,000 mail ballots were returned as of Sunday.

New York officials expect the vast majority of votes to be mail ballots this year, compared to their typical 5 per cent share. Counties have until eight days after Election Day to count and release the results of mail ballots, with 1.7 million requested by voters.

Other notable contests Tuesday included an effort by one-time CNBC anchor and former Republican Michelle Caruso-Cabrera to grab the Democratic nomination from Rep. Alexandria Ocasio-Cortez, the progressive icon who was an unknown 28-year-old when she won a 2018 primary over longtime congressman Joe Crowley.

As Tuesday's voting approached, President Donald Trump continued his effort to undermine Americans' faith in mail-in voting by repeating his unfounded claim that the system is rife with fraud.

"Because of MAIL-IN BALLOTS, 2020 will be the most RIGGED Election in our nations history - unless this stupidity is ended," he tweeted, accusing proponents of "using Covid in order to cheat."

Already this year, problems like long lines and lost mail-in ballots have plagued elections in Georgia, Nevada, Wisconsin and Pennsylvania.