WASHINGTON — Sen. Bob Casey on Thursday conceded a contentious and costly U.S. Senate race to Republican challenger David McCormick, marking the close of nearly 20 years in the Senate for one of Pennsylvania’s best-known politicians and sealing yet another loss for Democrats on Capitol Hill just as the second Trump administration is poised to come to power.

Announced in a speech Mr. Casey shared in a social media video Thursday evening, the concession came after mounting pressure from Republicans across the country who said the three-term Democrat had no chance of overcoming Mr. McCormick’s nearly 17,000-vote margin even with thousands of ballots embroiled in court and an automatic recount already underway.

“Thank you for your trust in me for all these years, Pennsylvania,” the characteristically reserved Mr. Casey, 64, said straight into the camera. “It has been the honor of my lifetime.”

According to the McCormick campaign, Mr. Casey had netted only seven additional votes after 16 counties had completed recounts thus far.