TV personality Dr. Mehmet Oz told supporters Tuesday night he was confident he'd come out on top.
"When all the votes are tallied, I am confident we will win."
But as of Tuesday night - out of nearly 1.3 million ballots counted - Oz was just 500 votes ahead of former hedge fund executive David McCormick.
This Republican primary is seen as another test of former president Donald Trump's sway over GOP voters.
"And I want to thank some other individuals who are actually unbelievably close friends who made a big difference in my life or are always there at every moment. Let's start with 45, President Trump."
Trump backed Oz over McCormick.
"I think we're seeing in Ohio and Pennsylvania, that there can be some gains probably made with the Republican primary electorate with the Trump endorsement, but it's not unlimited."
Marc Meredith teaches political science at the University of Pennsylvania. He says that while Trump-backed candidates may have won in a Republican Senate primary in Ohio, others, such as Trump's pick for Nebraska governor, fell short.
So too did his choice in North Carolina. There, the incumbent representative Madison Cawthorne was unseated following a string of scandals.
Meredith said Trump's impact in a May primary race could also play out differently in a November general election.
"There's going to be a decent set of voters for whom a Trump endorsement is going to be a very favorable thing, and another set of voters - even a few Republicans, probably, in Pennsylvania - for whom a Trump endorsement will not be seen as a good thing."
The outcome of the Senate race here in Pennsylvania could make or break Democrats' hopes of clinging to their razor-thin majority in the chamber.
Perhaps Democrats' best hope of flipping a seat lies in John Fetterman, Pennsylvania's lieutenant governor and the party's pick to run fo the open S enate seat.
Another race to watch will be the one for Pennsylvania's governor. The state's Democratic attorney general Josh Shapiro will face Trump-endorsed Republican Doug Mastriano.
Mastriano has parroted what's known as the Big Lie - the false claim that Democrats stole the 2020 presidential election through fraud - and Mastriano attended the January 6, 2021 rally that saw Trump supporters violently storm the U.S. Capitol.
Mastriano also supports a statewide ban on abortion if the Supreme Court overturns federal protections, as now seems likely.
Meredith: "As a Pennsylvanian, I have immense interest in this governor's race. You have two very contrasting candidates, that have very different visions of where Pennsylvania should be going and would do very different things in office."
Pennsylvania's governor appoints the secretary of state, who administers elections in the contested swing state that could sway the 2024 presidential race.
Meredith: "Any statewide election in Pennsylvania can be won by a Republican or a Democrat. It is such an evenly divided state, that, I think you have to go and, anytime someone wins a nomination, for one of the two major parties, and think that they do have a real chance to win."
