WASHINGTON (AFP) ― Mitt Romney on Tuesday effectively won the Republican Party crown to challenge President Barack Obama in November elections, as chief rival Rick Santorum dropped his long-shot White House bid.

“This presidential race is over for me,” Santorum told reporters in Gettysburg, in his home state of Pennsylvania.

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.

He trailed a distant second in the battle to be the party’s contender aiming to thwart Obama’s hopes of a second term, outspent by Romney’s deep war chest and out-organized by an extensive grassroots network.

Last 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 as he pledged to do what he can to help defeat the Democratic incumbent in November.

“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.”

In his speech, Santorum did not once mention Romney, who will clearly need Santorum’s backing if he is to corral the support of evangelical Christians and core conservative voters.

But Santorum spokeswoman Alice Stewart told CNN the pair had a “nice conversation” earlier today, and would have “another conversation in the next few days and weeks.”

The two have “great mutual respect for each other,” she said, while acknowledging that “politically, they disagree on some of the key issues.”

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

Santorum had already suffered a massive home defeat in 2006, when he was bounced out of the U.S. Senate by a Democratic challenger, and losing in Pennsylvania a second time would be a disaster for the political brand of a man many experts have said could be laying the groundwork for another run in 2016.

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

The Republican National Committee called Santorum’s decision to drop out “commendable.”

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.”

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

The 53-year-old was a virtual unknown nationally when he first threw his hat into the Republican ring in June.

Despite being written off early, Santorum had improbable successes in the U.S. South and Midwest.

“It was a love affair for me going from state to state,” he said Tuesday.

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

Romney, the millionaire ex-governor of liberal leaning Massachusetts, has also stirred controversy, with many saying he is out of touch with ordinary people struggling in the tough economy.

And Santorum was criticized for dwelling on contentious social issues at a time of economic hardship.

Pressure will now build on former House speaker Newt Gingrich to drop out and let the party coalesce around Romney.

But Gingrich insisted he will take his campaign to August’s Republican national convention despite little to no chance of winning enough delegates.

“I am committed to staying in this race all the way to Tampa so that the conservative movement has a real choice,” said Gingrich, who wasted no time in urging Santorum supporters to “join us.”

Gingrich, who has won two contests, acknowledged Sunday that Romney was his party’s “most likely” nominee and pledged to support him if Romney gets the 1,144 delegates to clinch the nomination.

Libertarian Texas congressman Ron Paul also issued a statement saying he planned to “continue running hard” until the party convention in Tampa.