Rick Santorum abruptly suspended his White House campaign Tuesday, clearing the way for front-runner Mitt Romney to claim the Republican presidential nomination.

The former Pennsylvania senator made the announcement at a news conference Tuesday afternoon. Mr. Santorum significantly lagged Mr. Romney in the nominating contest and faced an increasingly hard challenge in winning the GOP primary in his home state of Pennsylvania, which votes later this month.

Mr. Santorum was the last viable challenger to Mr. Romney, and his decision to exit the race marks the unofficial end of the Republicans’ months-long, highly combative primary. Mr. Romney has yet to gather the 1,144 convention delegates needed to win the nomination, but it is unlikely that the remaining challengers, Newt Gingrich and Rep. Ron Paul, can prevent him from doing so.

Despite the long odds facing his campaign, Mr. Santorum had promised to stay in the race until Pennsylvania’s April 24th primary, but polls show his support in the state sagging, and Mr. Romney promised to outspend him significantly on television ads in the state.