Celebrity doctor Mehmet Oz and former hedge fund executive Dave McCormick are neck and neck. Political commentator Kathy Barnette trails behind by more than 86,000 votes.
As of Wednesday morning, with 95% of precincts reporting, Oz had 411,674 votes while McCormick held 409,002 votes. Barnette trailed behind with a total of 325,557 votes.
Pennsylvania law requires recounts when the margin between the top two candidates is 0.5 percentage points or less. With just a few hundred votes separating Oz and McCormick, the result may not be clear for several days.
Election Results: Live updates on Pennsylvania primary races
The winner will face Democratic challenger John Fetterman, who won his party's nomination days after suffering a stroke.
"We're not gonna have a result tonight," Oz said shortly before midnight, before vowing to former President Donald Trump, "I will make you proud."
Oz had been locked in an expensive battle with McCormick. But Barnette, who has drawn the support of Trump backers suspicious of Oz's ideological shifts, stunned the political world with a late surge that upended the race in the final weeks as she tries to become the first Black Republican woman elected to the Senate.
Barnette, who voted in Huntingdon Valley on Tuesday morning, has repeated false claims the 2020 election was stolen.
In recent days, pictures have emerged of Barnette apparently marching near members of the Proud Boys on Jan. 6, 2021. ABC News has verified the images that were first shared by an independent researcher.
She denied any connection to the Proud Boys to another network.