Senator Bob Casey, a three-term Pennsylvania Democrat, on Thursday conceded to his Republican rival, David McCormick, amid a recount in their marquee Senate contest.

The New York Times has not yet called the race, which went to a recount because Mr. McCormick led Mr. Casey by less than half of a percentage point in an initial tally. The post-election period set off legal battles between the two sides.

“This race was one of the closest in our commonwealth’s history,” Mr. Casey said in a statement. “I am grateful to the thousands of people who worked to make sure every eligible vote cast could be counted.”

A former hedge fund executive, Mr. McCormick channeled the sour national mood into what appears set to be a stunning victory over a well-established incumbent.

Mr. Casey, a mild-mannered Scranton native and a son of a popular former governor of the state, is a Pennsylvania institution. He ran as a populist, lashing big corporations for what he called “greedflation” and promising to fight for middle- and working-class Americans.

But he faced increasingly difficult headwinds as the presidential race tightened.

Mr. McCormick, a West Point graduate who lost the G.O.P. Senate primary in 2022, found his footing in seeking to tie Mr. Casey to Vice President Kamala Harris in the closely divided state. In particular, he highlighted her past support for banning fracking, though she no longer holds that view and Mr. Casey opposes fracking bans.