LOUISVILLE, Ky. — Former Marine pilot Amy McGrath overcame a bumpier-than-expected Kentucky primary to win the Democratic U.S. 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 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 McGrath. With 99 percent of precincts reporting Tuesday afternoon, she had an 11,832-vote advantage over Booker out of nearly 531,000 votes cast. Several other candidates attracted tens of thousands of votes. 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, McConnell and McGrath looked past their primaries to skirmish with each other, and now those attacks will intensify heading into the fall campaign.