The Associated Press and other outlets earlier on Thursday projected McCormick as the winner over Casey.
The news agency said it called the race because there were not enough outstanding votes from areas that Casey was winning for him to end up as the victor.
With so many ballots left to count, Casey said that he would not yet concede to his opponent.
If the projected win holds, Republicans would have at least 53 seats in the upper chamber in the next Congress after spending four years in the minority.
Races in Nevada and Arizona have yet to be called.
Casey, 64, defeated incumbent Sen. Rick Santorum (R-Pa.) in 2006 and won reelection in 2012 and 2018. This year was the first time he was on the same ballot as President-elect Donald Trump, for whom a majority of Pennsylvanians voted.
