Incumbent Dems face a reckoning on primary day: Will progressives unseat them?

New York's coronavirus-delayed primary is Tuesday — and it has the potential to throw a wrench into the power structure in the Democratic-controlled House — while in Kentucky there's a Democratic battle for the right to challenge Senate Majority Leader Mitch McConnell in November.

In New York, among those fighting primary challenges from their left are Democratic Reps. Eliot Engel, chair of the Foreign Affairs Committee; Carolyn Maloney, chair of the Oversight Committee; and Jerry Nadler, chair of the Judiciary Committee — all longtime incumbents.

Of the powerful trio, Engel, who's in his 16th term in Congress, is by far the most vulnerable. His race has become a flashpoint in the battle between establishment Democrats and the progressive wing of the party.

Democratic Rep. Alexandria Ocasio-Cortez, who represents the district next to Engel's, has endorsed his more progressive challenger, Jamaal Bowman, a 44-year-old father of three and former middle school principal who has campaigned on racial injustice and human rights.

Other progressives have followed Ocasio-Cortez's lead, with her fellow "Squad" member, Rep. Ayanna Pressley of Massachusetts, and Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts, both of them former presidential candidates, endorsing Bowman, as well.

Engel has countered with his own big-name backers, including former presidential nominee Hillary Clinton, Senate Minority Leader Chuck Schumer of New York, House Speaker Nancy Pelosi of California, New York Gov. Andrew Cuomo and Rep. John Lewis, D-Ga., the civil rights leader.

"If veteran congressman Eliot Engel falls, the Democratic Party will be apoplectic," longtime New York political strategist Hank Sheinkopf told NBC News. "They'll have to deal much more with the AOC wing of the party. The left wing of the party will be better positioned, and Joe Biden will have a bigger headache the next morning."

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Maloney and Nadler appear to be on much safer ground, Sheinkopf said.

Maloney is running against three challengers, including Suraj Patel, whom she defeated in a one-on-one contest in 2018. Nadler is running against two challengers who ripped him as "all talk" on progressive issues during a NY1 debate last week, arguments that he countered by pointing to the endorsements he's gotten from Ocasio-Cortez and Warren.

Ocasio-Cortez, meanwhile, is defending her own seat from a challenger who has been using some of her own tactics against her.

Ocasio-Cortez won the seat in 2018 in a major upset over Joe Crowley, one of the top-ranking Democrats in the House. She'd argued that he spent too much away from the district focused on issues that didn't involve his constituents — the same complaints her main challenger, Michelle Caruso-Cabrera, has made against her by saying "AOC is MIA."

Because of the pandemic, Caruso-Cabrera, a former CNBC anchor, hasn't been able to knock on as many doors as Ocasio-Cortez did in 2018, and Ocasio-Cortez has a significant money advantage, having raised over $10 million. That's five times more than Caruso-Cabrera has raised.

Primary day will also include a special election delayed from April to fill the upstate New York seat left vacant by the resignation of Republican Rep. Chris Collins, who pleaded guilty to insider trading last year.

The pandemic is looming over the primary in other ways.

Cuomo issued an executive order in April calling for absentee ballot applications to be sent out to all voters in the state in a bid to make voting during the pandemic safer. In New York City, the city Board of Elections has sent 679,000 ballots to voters who've requested them.

Statewide, over 1.6 million absentee ballots have been sent out, according to the state Board of Elections. In 2016, 115,000 people voted absentee in the presidential primary.

What the surge in absentee voting means for the primary is unclear, Sheinkopf said.

"Nobody knows what the outcomes will be, because there's no previous barometer for this in New York," he said. "The answers are all in the mailboxes."

Under state law, absentee ballots can't be counted until one week after the election. And as for how long it will take to count all those ballots, city Board of Elections spokeswoman Valerie Vazquez-Diaz said, "We don't know."

Tuesday is also primary day in Kentucky, where Democrats will select a candidate to take on McConnell in the fall.

Former fighter pilot Amy McGrath had been the favorite, with strong fundraising numbers and support from establishment Democrats. But over the last few weeks, rival Charles Booker, a progressive state representative, has picked up steam — and the endorsements of Ocasio-Cortez and Sanders.

Booker has campaigned on racial injustice and inequity, and he has taken to the streets with Black Lives Matter demonstrators to protest the killing of Breonna Taylor, a Louisville woman who was shot dead in her apartment on March 13 by police executing a "no-knock" warrant.

He has also made an issue of McGrath's failure to protest — leading her to air an ad decrying the death of George Floyd. Booker noted that she didn't mention Taylor in the ad.

As in New York, there's been a surge in absentee voting in Kentucky, with Gov. Andy Beshear regularly pleading for voters to vote by mail.

It may be hard for them to do otherwise — only 200 polling places are expected to be up and running on primary day, instead of the usual 3,500, which could lead to substantial voting problems and long waits Tuesday. The state has cited a shortage of available poll workers amid coronavirus safety concerns for the drastic reduction.

Voting rights experts say that while mail-in voting is key to reducing density at polling places, election administrations should expect that a significant number of people will still want to vote in person. Georgia's primary, for instance, had a surge of absentee ballot applications, but droves of voters still showed up to vote in person — either out of choice or because absentee voting applications or ballots never arrived.

According to a court filing after advocates unsuccessfully sought to add polling sites last week, Jefferson County, which includes Louisville, is one of several counties with just one in-person polling location.

As of last week, 202,652 absentee ballots had been requested in Jefferson County, suggesting a strong appetite for absentee voting, but those who want to vote in person on Election Day will have just one place to do it: the Kentucky Exposition Center. The Kentucky Exposition Center is quite large, with 1.3 million square feet of indoor space and more than 19,000 parking spaces. It will be set up to have 420 voting booths, according to the filing.

But if turnout is high, the center could easily have long lines: To serve just a quarter of the county's registered, eligible voters in person, roughly 213 voters would need to vote every single minute continuously for 12 hours straight.