By Jarrett Renshaw

PHILADELPHIA David McCormick, a former hedge fund executive who lost the Republican primary for Pennsylvania's open Senate seat last year, will announce on Thursday that he is running again for Senate — this time against three-time incumbent Democratic Senator Bob Casey Jr.

A U.S. Army veteran and former Treasury Department official, McCormick, 58, will kick off one of the nation's most closely watched races with a speech in Pittsburgh, two sources said. Reuters first reported on Sept. 12 that the former CEO of Bridgewater Associates, one of the world's largest hedge funds, would run.

Pennsylvania, one of the nation's most politically competitive states, has tilted toward Democrats in recent years. Republicans in 2022 faced several losses in the state, including candidates for Senate and governor backed by former President Donald Trump.

Republicans view the seat as a potential game-changer in 2024 for control of Congress. Democrats are defending a one-seat Senate majority while facing tough incumbent races in West Virginia, Ohio and Montana - states who backed Donald Trump and other Republicans in recent elections.

The Cook Political Report, a nonpartisan newsletter, describes the race McCormick is joining as leaning Democratic.

U.S. Senate Republicans focused on McCormick as they try to line up fresh candidates for 2024, after newcomers with fringe views cost them seats in 2022. Republicans believe he is moderate enough to appeal to suburbanites and stable enough to help other candidates on the ticket.

Pennsylvania had a Republican senator and a Democratic senator for more than a decade until Republican Patrick Toomey retired in January. U.S. Senator John Fetterman, a Democrat, picked up the Toomey seat last year after beating television celebrity physician Mehmet Oz.

The McCormick campaign did not respond to requests for comment.

McCormick's decision comes after weeks of wrangling over whether Trump at the top of the ticket could be a drag, making it more difficult to oust the well-funded Casey, 63. The popular sitting Democratic senator, whose father was a Pennsylvania governor, has won three state-wide election.

McCormick is already facing attacks from Democrats about whether he truly lives in Pennsylvania. He changed his residency from Connecticut last year to run for office, but The Associated Press reported last month that McCormick still rents a $16 million mansion in Connecticut.

(Reporting by Jarrett Renshaw. Editing by Heather Timmons and Lincoln Feast.)