WASHINGTON  - Rick Santorum Tuesday dropped his long-shot White House bid, effectively handing Mitt Romney the Republican party crown to challenge President Barack Obama in November elections.

"This presidential race is over for me," Santorum told supporters in Gettysburg, in his home state of Pennsylvania.

Despite an upset victory when he won the very first contest in the Republican nominating race in Iowa in January, Santorum has failed to build any momentum.

He has trailed a distant second in the battle to be the party's contender aiming to thwart Obama's hopes of a second four-year term, outspent by Romney's deep war chest and out-organized by his extensive grass-roots network.

Over the Easter weekend, the former Pennsylvania senator halted campaigning for four days after his youngest daughter, Bella, who suffers from a rare genetic disorder, was hospitalized again.

Santorum acknowledged it had been a "difficult weekend," and said while Bella was getting better, "it did cause us to think."

"While this presidential race is over for me and we will suspend our campaign effective today, we are not done fighting," he said. "We're going to continue to fight for the Americans who stood up and gave us that air under our wings that allowed us to accomplish things that no political expert would have ever expected."

His surprise decision to quit the race came just two weeks before a primary vote in Pennsylvania, amid polls showing he could lose the state to Romney.

With Romney enjoying a commanding lead in the delegates race that determines the nominee, the party establishment had been urging Santorum to step aside and allow the party to coalesce around one single candidate.

Romney swiftly congratulated Santorum for being "an able and worthy competitor" and for the campaign he had run.

"He has proven himself to be an important voice in our party and in the nation," Romney added.

A devout Catholic, Santorum's radical views on religion, women and marriage played well with the party's conservative base, but were alienating the all-important voting bloc of moderates and independents.

The 53-year-old former Pennsylvania senator was a virtual unknown on the national scene when he first threw his hat into the ring in June to be the Republican nominee.

Despite being written off early on, Santorum built his campaign state-by-state, winning key successes in some of the southern conservative states.

His pro-life, anti-contraception, marriage-only-between-a-man-a-woman message gained traction with heartland evangelicals deeply skeptical of Romney, who they view as a moderate disguised in conservative clothing.

Romney, the multi-millionaire former governor of the liberal East Coast state of Massachusetts, has also stirred controversy, with many saying he is out of touch with ordinary people struggling in the tough economy.

He has also drawn suspicion because of his Mormon faith.

Pressure will now build on former House speaker Newt Gingrich to pull out and allow the party to heal from the divisive campaign and come together around Romney ahead of the November 6 presidential polls.

Gingrich, who has won just two contests and is also lagging far behind his rivals with his campaign mired in debt, acknowledged Sunday that Romney was his party's "most likely" nominee.

But he insisted he was staying in the race despite being far outpaced, after polls once had him as the leading Republican to take on Obama.

"I think you have to be realistic, given the size of his organization, given the number of primaries he's won. He is far and away, the most likely Republican nominee," Gingrich told "Fox News Sunday."

If Romney gets the 1,144 delegates to clinch the nomination at the party's August convention in Tampa, Florida, Gingrich said: "I'll support him. I'll do everything I can this fall to help him defeat Obama."

Libertarian Texas congressman Ron Paul also remains in the race but has no hope of winning the party's nomination.