Roughly 27,000 votes separate McCormick and Casey.
Secretary of the Commonwealth Al Schmidt said the vote totals fall within the one-half of 1 percent margin, which triggers a mandatory recount under state law.
Once counties finish counting their ballots, they must begin the recount no later than Wednesday, Nov. 20. They must complete the recount by noon on Nov. 26 and must report results to the Secretary by noon on Nov. 27.
The results of the recount will not be published until Nov. 27.
McCormick declared victory last week a day after the Associated Press announced a winner in the race.
READ MORE: McCormick declares victory in PA Senate race, sues over provisional ballots in Philadelphia
ABC News has not yet projected a winner.
Casey, perhaps Pennsylvania's best-known politician and the son of a former two-term governor, is seeking a fourth term after facing what he has called his toughest reelection challenge yet. Casey, 64, has won six statewide elections going back to 1996, including serving as the state's auditor general and treasurer.
McCormick, 59, is making his second run for the Senate after losing narrowly to Dr. Mehmet Oz in 2022's Republican primary. He left his job as CEO of the world's largest hedge fund to run after serving at the highest levels of former President George W. Bush's administration and sitting on Trump's Defense Advisory Board.
Even after a recount, experts believe a Casey comeback is unlikely.
"We certainly believe in counting every single vote and making sure we have an accurate count, but no one here anticipates that the results will change," said Lauren Cristella, president & CEO of Committee of Seventy, a nonprofit, non-partisan group that educates and advocates for voters.
As of now, Schmidt says there are just over 80,000 provisional, mail-in and absentee ballots outstanding. This includes ballots for which county boards of elections have not determined their validity or eligibility. The ballot curing deadline for voters was Tuesday.
The recount is estimated to cost taxpayers more than $1 million.