If McGrath wins — most Democrats tracking the race think she remains the favorite — it will be another bitter defeat for the left wing of the party, which is also seeking victories in several House primaries Tuesday. Progressives failed to mount serious challenges to the party establishment in most Senate races in recent cycles and lost in the party’s presidential primary.

If Booker manages to pull it off, it would be the first primary defeat for a candidate backed by the Democratic Senatorial Campaign Committee in a decade.

Had the election been in May — it was postponed a month because of coronavirus — “I don’t think Charles had a snowball’s chance,” said Greg Stumbo, a Democratic Party fixture in Kentucky who has served as state attorney general and state House speaker. “But I think he’s got a chance. He’s got some momentum. He’s got excitement.”

Stumbo, who spoke at a rally for Booker in one of the state’s rural counties on Monday, said he’s been hearing from more Democrats supporting Booker since endorsing him last week.

“He’s in the right place at the right time. Whether that’s enough to overcome all the money she’s been putting into advertising and the name recognition she has, and all the people they say who voted before Charles really took off ...” Stumbo said. “I don’t think anybody can predict this one.”

McGrath’s advisers and allies remain confident. Her campaign had been polling the race consistently, but stopped its daily tracking after Thursday because of confidence her numbers were stable, Mark Nickolas, her campaign manager, told POLITICO. The polling had shown her consistently up by between 10 and 13 percentage points.

But they're not taking it for granted: Her campaign has increased its advertising in the final days, spending roughly $3 million over the past week across the state, including media markets in which Booker cannot afford to advertise. Recently, she's run a message focused more squarely on Democratic voters than some of her previous ads targeting a general election audience.

“The way you win Kentucky is you have to have an operation that can win in 120 counties, especially those rural and conservative counties,” Nickolas said.

McGrath and her allies have also focused on the argument that she is best-suited to take on McConnell in the fall. President Donald Trump is likely to carry the state overwhelmingly, and significant crossover votes would be necessary to defeat McConnell, even though polls show he is unpopular among most Kentucky voters. Booker’s support of "Medicare for All" and a "Green New Deal" would give McConnell easy attacks to paint him as a radical.

McGrath is leaning into the disdain for McConnell among Democrats: In her latest ad, which began airing over the weekend, a narrator called her "Kentucky’s best chance to move on from Mitch McConnell."

In an interview, Booker defended his support of more liberal policies and argued they would not be a detriment in a general election. He said he was “redefining electability.”

“We're going to shock the world. And after we do, I look forward to building a new coalition of a lot of folks that didn't give us a chance, so that we can go ahead and fight for our future for everyone in this country,” Booker said.

He also brushed aside the massive resources gap in the race. McGrath had already raised tens of millions of dollars by the time Booker announced he was running in January, six months after she launched last summer.

“We were never afraid of the big money,” Booker said. “That was really the whole point. We knew that money doesn't measure up to people."

But McGrath’s fundraising prowess would be a major advantage for Democrats in the fall, forcing Republicans to keep an eye on Kentucky: A super PAC with close ties to the majority leader reserved $10.8 million in the Bluegrass State for the fall, alongside reservations in more obvious battlegrounds like Arizona, North Carolina and Maine.

McGrath's campaign has spent more than $12 million on TV so far, compared with just $1.3 million for Booker, who did not start advertising on TV until the second week in June, according to data from Advertising Analytics.

One major factor creating uncertainty is voting changes because of the pandemic. The state allowed all voters to request absentee ballots but also limited in-person voting stations Tuesday, including just one large polling location in Jefferson County, the state's most populous county and home to Booker's state House district in Louisville.

More than 860,000 absentee ballots were issued, according to the secretary of state’s office, and more than 440,000 were returned across the state — and at least 88,000 voters voted in person during the period of early voting. But long wait times on Tuesday remain possible, and it’s unlikely that a result will be called on Tuesday because not all absentee ballots will have been received.

Regardless of who emerges as the nominee, defeating McConnell will clearly be an uphill climb in a state that hasn’t elected a Democrat to the Senate in three decades. But the approaches would be very different.

“If you're McGrath, you have plenty of resources and the ability to raise more money with the push of a button,” said Mark Riddle, a veteran strategist in the state. “And if you're Booker, you feel like movement politics has never really been done against Mitch McConnell before, and maybe that's the key.”