Rick Santorum who defied the odds to become the standard-bearer of the Republican Right, said he had decided to end an unlikely campaign for the White House

Rick Santorum, 53, who defied the odds to become the standard-bearer of the Republican Right, said he had decided to end an unlikely campaign for the White House, just two weeks before a primary vote in his home state of Pennsylvania, with polls suggesting he could lose to Romney there.

Despite an upset victory in the first Republican nominating contest in Iowa in January that led to a surprising showing with victories in 10 more states, Santorum failed to build sufficient momentum to derail frontrunner Romney.

Santorum's departure saves him from the humiliation of likely defeat to Romney in Pennsylvania, his home state's primary later this month, and keeps alive his hopes of another White House bid in the future.

Mitt Romney, who has already turned his focus to Barack Obama, congratulated Santorum for being "an able and worthy competitor" who had "proven himself to be an important voice in our party and in the nation", adding: "I congratulate him on the campaign he ran," The Telegraph reported late Tuesday.

"While this presidential race is over for me, we are not done fighting. We are going to continue to fight for those Americans who stood up and gave us air under our wings that allowed us to accomplish things that no political expert would have ever expected," Santorum told a press conference in Gettysburg, Pennsylvania.

In his speech, Santorum did not mention Romney, who may need Santorum's backing to corral the support of evangelical Christians and core conservatives.

Santorum had already suffered a massive home defeat in 2006, when he was bounced out of the US Senate by a Democratic challenger.

For its part, the Republican National Committee called Santorum's decision to drop out "commendable," while Newt Gingrich, a former House Speaker, and Ron Paul, a Texas congressman pledged to remain in the race until the convention.

However, pressure will now build on Gingrich to drop out and let the party coalesce around Romney, but he has insisted he will take his campaign to August's Republican national convention despite little to no chance of winning enough delegates.

Gingrich, who has won just two state contests, acknowledged Sunday that Romney was his party's "most likely" nominee and pledged to support him if Romney gets the 1,144 delegates needed to clinch the nomination, AFP reported.

Libertarian Texas congress man Ron Paul said he planned to "continue running hard" until the party convention in Tampa.

On the other hand, Obama's campaign manager Jim Messina meanwhile used the opportunity to take aim at Romney, saying: "It's no surprise that Mitt Romney finally was able to grind down his opponents under an avalanche of negative ads."