Former President Donald Trump has urged celebrity doctor Mehmet Oz to declare victory in Tuesday's too-close-to-call Pennsylvania Republican Senate primary, even as ballots keep dribbling in.
With about 95% of votes counted, Dr Oz leads with 31.3% of the vote, compared to 31.1% for challenger Dave McCormick.
State law says a recount must be held if a candidate's margin of victory is under 0.5%.
Thousands of mail-in ballots must still be counted.
On Wednesday, Mr Trump took to Truth Social - his new social media platform - to cast doubt on the election and claim that Dr Oz had won.
In another post, he urged Dr Oz, whom he has endorsed, to "declare victory".
"It just makes it harder for them to cheat with the ballots that they 'just happened to find'," he wrote, providing no evidence to back up his claims.
In Lancaster County, a ballot print error meant that thousands of mail-in ballots had to be reprocessed.
The race between Dr Oz and Mr McCormick, a former hedge fund CEO, will determine who will run as the next Republican senator for Pennsylvania in the forthcoming mid-term elections.
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Midterms, which take place two years into a president's term, decide who controls the Senate and House of Representatives, the two legislative chambers that collectively make up Congress.
The winner of the primary will face off against John Fetterman, the state's lieutenant governor who overwhelmingly won the Democratic nomination despite suffering a stroke just days ago.
Pennsylvania is considered a key battleground state in the upcoming midterms. The Senate seat is currently held by a Republican, Patrick Toomey, who is retiring.
He compared the state's counting of mail-in ballots to the 2020 election, which he continues to insist he won.
On Tuesday, both candidates said they were confident of winning. A third candidate, Kathy Barnette, trails at 24.7% after a last-minute surge.
"We can see the path ahead," Mr McCormick told supporters. "We can see victory ahead."
Mr Oz, for his part, thanked both Mr Trump and a Fox News television host for their support of his campaign.
"President Trump after he endorsed me continued to lean into this race in Pennsylvania," Mr Oz said. "God bless you sir, for putting so much effort into this race. I will make you proud."
Under Pennsylvania law, a recount must be ordered by the afternoon of the second Thursday after the election - which in this case would be 26 May. The recount would then have to be held by 1 June.