McGrath won the Kentucky primary. Next, she'll face McConnell.

Former Marine pilot Amy McGrath overcame a bumpier-than-expected Kentucky primary to win the Democratic United States Senate nomination Tuesday, fending off progressive Charles Booker to set up a bruising, big-spending showdown with Republican Senate Majority Leader Mitch McConnell.

Voting ended June 23, but it took a week until McGrath could be declared the winner due to the race's tight margins and a deluge of mail-in ballots. The outcome seemed a certainty early in the campaign but became tenuous as Mr. Booker's profile surged as the Black state lawmaker highlighted protests against the deaths of African Americans in encounters with police.

It was a narrow victory for Ms. McGrath. She outlasted Mr. Booker by 15,149 votes out of more than 544,000 votes cast. Several other candidates attracted tens of thousands of votes. Mr. McConnell, a key ally to President Donald Trump, already breezed to victory in the GOP primary in his bid for a seventh term.

Kentucky switched to widespread absentee voting amid the coronavirus pandemic, and election officials needed days to count ballots. In Lexington, the state's second-largest city, about 6,000 absentee ballots were thrown out on technicalities ranging from unsigned envelopes to detached security flaps, said Fayette County Clerk Don Blevins.

Since last summer, Mr. McConnell and Ms. McGrath had looked past their respective primaries to skirmish with each other. Those attacks will now intensify heading into the fall campaign.

Declaring victory, Ms. McGrath reached out to Mr. Booker's supporters to try to unite the party for the challenge ahead against Mr. McConnell, who has dominated Kentucky's political landscape for decades.

"There is far too much at stake," Ms. McGrath said in a statement. "The differences that separate Democrats are nothing compared to the chasm that exists between us and the politics and actions of Mitch McConnell. He's destroyed our institutions for far too long."

Mr. McConnell's campaign said Ms. McGrath's narrow victory showed her candidacy is damaged heading into the general election.

Mr. McConnell campaign spokeswoman Kate Cooksey portrayed Ms. McGrath as a "tool" for the national Democratic establishment and said the challenger was out of step with Kentucky with her support for abortion rights and "government-run health care."

Mr. Booker conceded later in the day and called on Democrats to dedicate themselves "to the work of beating Mitch, so ... we can get him out of the way." But his statement mentioned Ms. McGrath only briefly, focusing instead on his campaign for economic and racial justice.

"We've proven Kentuckians are hungry for a new kind of leadership, one that puts working people and their struggles before corporate special interests and the corrupt politicians who serve them," he said. "We've proven you don't have to pretend to be a Republican to run as a Democrat in Kentucky, and that people want big, bold solutions to the enormous crises our state is facing."

Ms. McGrath has raised prodigious amounts of campaign cash, capitalizing on the disdain national Democrats have for Mr. McConnell. It places her in a position to go toe-to-toe with the always-well-funded Mr. McConnell.

Despite her advantages, Ms. McGrath sweated out her victory against the hard-charging Mr. Booker.

Mr. Booker's long-shot bid surged amid the national eruption of protests against police brutality. He joined demonstrations in his hometown of Louisville to demand justice for Breonna Taylor, who was fatally shot by Louisville police in her own home. Mr. Booker gained the backing of leading national progressives as he supported a universal basic income and Medicare for All — ideas that Ms. McGrath resisted.

Ms. McGrath charted a more moderate course inside Democratic politics. She supports adding a public health insurance option as part of the Obama-era Affordable Care Act and supports expanded access to Medicare for people 55 and older.

She portrays Mr. McConnell as an overly partisan, Washington insider who exemplifies what's wrong with national politics. She accuses Mr. McConnell of undermining labor unions, awarding tax cuts for the wealthy, and cozying up to pharmaceutical companies while people struggle to afford prescription drugs.

Mr. McConnell accuses her of being too liberal for Kentucky on issues ranging from abortion to border security. He promotes his work with Mr. Trump – who remains popular in Kentucky – to appoint conservatives to fill federal court seats. Mr. McConnell also plays up his Senate leadership role and his ability to steer federal money back to the Bluegrass State.

Mr. Trump could turn into a focal point in the Senate race.

Mr. McConnell led the effort to defend the president after House Democrats impeached him. Ms. McGrath has said she would have voted to convict Mr. Trump on both impeachment counts. She accused of the GOP-led Senate of lacking "the guts" to put a check on "out-of-control presidential power."

This story was reported by The Associated Press. AP writer Piper Hudspeth Blackburn contributed to this report.

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