Republican Dave McCormick won the Senate race in Pennsylvania on Thursday, denying Democratic incumbent President Bob Casey a fourth term and expanding his party’s majority in the Senate.

When the Associated Press called the race at 4:09 PM ET on Thursday, two days after polls closed in Pennsylvania, McCormick led by 0.4 points. The slim margin raises the possibility of a recount, although his victory is expected to hold given his lead of about 30,000 votes.

A spokesperson for Casey insisted that thousands of ballots remained uncounted and refused to concede the race to McCormick.

“As Pennsylvania’s Secretary of State said this afternoon, there are tens of thousands of ballots remaining to be counted across the Commonwealth, including provisional ballots, military and overseas ballots, and mail-in ballots,” Casey spokesperson Maddy McDaniel said in a statement. “This race is within a half-point and cannot be called while the votes of thousands of Pennsylvanians are still being counted. We will ensure that every Pennsylvanian’s voice is heard.”

With McCormick’s victory, Republicans have now secured at least 53 seats in the Senate, wiping out Democrats’ previous majority in the House. Two Senate races in Nevada and Arizona remained too close to call Thursday afternoon.

Although he fell short, Casey outperformed Kamala Harris, who lost Pennsylvania to Donald Trump by two points. Trump also won the two other “blue wall” states of Michigan and Wisconsin, but Democrats managed to hold on to both Senate seats that were up for grabs in those states.

The call in Pennsylvania ended a contentious and expensive Senate race in which the two candidates traded sharp attacks over the cost of living, access to abortion and McCormick’s recent residency in Connecticut. Casey attacked McCormick, a former hedge fund CEO, for being out of touch, while McCormick linked Casey to the Biden administration’s “reckless” government spending.

During their debate last month, Casey derided McCormick as “bought and paid for by these billionaires and corporations.” McCormick fired back, saying, “If you don’t have a record to build on, which Senator Casey doesn’t, you attack your opponent.”

The high stakes of the race made it one of the most expensive Senate elections in the country, as the dueling campaigns and their allies spent more than $300 million on advertising. One pro-McCormick organization, the Keystone Renewal Pac, spent at least $54 million on the race, making the group the highest-spending single-candidate Pac involved in a Senate race this election cycle.

Most of the race’s public polls showed Casey leading by several points in recent weeks, as McCormick narrowed that gap to just a few points. Despite that trend, Casey appeared to be in a slightly stronger position than Harris, who was neck and neck against Trump in Pennsylvania until Election Day. Leaders from both parties had identified Pennsylvania and its 19 electoral votes as the potential tipping point in the presidential race.

“I think both races will be very close, but I think the people of our state know it’s a very, very clear choice,” Casey told the Guardian in September. “It has never been clearer.”

Before Election Day, Democrats had a 51-49 majority in the Senate. Republicans’ victories in Montana, Ohio and West Virginia had already guaranteed control of the Senate, but McCormick’s victory will give the party even more power to implement Trump’s agenda when the new Congress convenes in January.

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