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Rick Santorum suspended his presidential campaign on Tuesday, bowing to the inevitability of Mitt Romney ’s nomination and ending his improbable, come-from-behind quest to become the party’s conservative standard-bearer in the fall.

“We made a decision over the weekend, that while this presidential race for us is over, for me, and we will suspend our campaign today, we are not done fighting,” Mr. Santorum said.

Mr. Santorum made the announcement at a stop in his home state of Pennsylvania after a weekend in which he tended to his 3-year-old daughter, Bella, who had been hospitalized with pneumonia.

Mr. Santorum, who was holding back tears, did not exactly specify why he was ending his presidential bid. He referred to his daughter’s illness, but said she was making great progress and was back home after being hospitalized over the weekend.

Mr. Santorum called Mr. Romney earlier in the day to tell him of his plans to suspend his campaign. Mr. Santorum told Mr. Romney that he is committed to defeating President Obama, but that he is not going to endorse immediately, said a source familiar with the call.

Mr. Santorum made no mention of Mr. Romney in a 12-minute speech in which he extolled the people he had met during the campaign and said he was inspired by their stories of struggle and faith.

After spending months accusing Mr. Romney of being a weak challenger for President Obama, Mr. Santorum said only that he would continue to fight to elect a Republican president and to ensure Republican control of Congress.

“This game is a long, long, long way from over,” he said.

Mr. Romney issued a statement moments after Mr. Santorum concluded his remarks.

“Senator Santorum is an able and worthy competitor, and I congratulate him on the campaign he ran,” Mr. Romney said. “He has proven himself to be an important voice in our party and in the nation. We both recognize that what is most important is putting the failures of the last three years behind us and setting America back on the path to prosperity.”

As recently as December, Mr. Santorum was operating a shoestring campaign in Iowa, traveling with just a handful of aides in a pickup truck. But his brand of conservative populism caught fire in Iowa, where he defeated Mr. Romney. And then it caught fire again in several Midwest primaries in which he surprised Mr. Romney.

But ultimately, Mr. Santorum’s campaign struggled under a nearly constant barrage of negative ads paid for by Mr. Romney and the “super PAC” supporting him, Restore Our Future, which has spent millions in an effort to ensure that Mr. Romney captures the nomination in his second attempt.

Mr. Santorum’s withdrawal abruptly upends not only the political calculus for Mr. Romney and his campaign team, but also their financial picture.

Both candidates had faced fund-raising challenges in the coming weeks, with Mr. Santorum limping by on fumes and Mr. Romney rapidly tapping his top contributors for the maximum contribution, challenging him to seek out new sources of cash. Mr. Romney had committed $2.9 million to the Pennsylvania primary, hoping to deliver Mr. Santorum a knockout blow, and Restore Our Future had begun spending there as well.

Mr. Santorum’s withdrawal will allow both Mr. Romney and the super PAC to redirect that money to other states, either to head off any lingering challenge from Newt Gingrich and Representative Ron Paul of Texas, or to begin preparing for what seems to be an increasingly likely general election matchup against Mr. Obama.

A former congressman and senator from Pennsylvania, Mr. Santorum had built a reputation as an unwavering social conservative whose opposition to abortion and same-sex marriage helped catapult him into national office.

Among the people who had been publicly encouraging Mr. Santorum to reconsider a drawn-out challenge to Mr. Romney were some evangelical leaders who had been suspicious of Mr. Romney’s commitment to their most important causes.

Richard Land, the president of the ethics commission at the Southern Baptist Convention, said as much an hour before word of Mr. Santorum’s decision reached the press.

“As his friend, I would say, you know you’ve done an incredible job resurrecting your career. You’ve done better than anybody thought you could,” Mr. Land told reporters and editors of The New York Times.

Mr. Land said that if Mr. Santorum pressed ahead, he would jeopardize that success. And he said that Mr. Santorum had a good future if he acknowledged Mr. Romney’s claim to the nomination this year.

“In eight years, Rick Santorum will be three years younger than Romney is now. He’s only 53 years old,” Mr. Land said. “He’ll be a significant player. I would think he could have a significant role in a Romney administration if he wanted to. Maybe H.H.S. secretary?”

Ralph Reed, a leading social conservative who oversees the Faith and Freedom Coalition, said Mr. Santorum had proved himself as the strongest insurgent conservative candidate since Ronald Reagan in 1976.

“It was an impressive performance and it leaves him with an elevated status and a prominent role as a leader for evangelicals and conservatives,” Mr. Reed said on Tuesday. “No one can know what the future holds, but my guess is we haven’t heard the last from Rick Santorum.”

Mr. Santorum’s candidacy benefited from the comparison to Mr. Romney as the Republican candidates appealed to a conservative segment of the Republican Party during the primary process. Mr. Santorum regularly mocked Mr. Romney as a flip-flopper on social and conservative issues who could not be trusted.

That helped Mr. Santorum win several Southern primaries in which evangelical voters and Tea Party supporters dominated the primary electorate.

But Mr. Santorum also cast himself as the true economic conservative who understood the needs of the middle class. His campaign attacked Mr. Romney, a multimillionaire, as out of touch with the needs and interests of regular working Americans.

Mr. Santorum’s quick rise in the polls also led to repeated gaffes that knocked him off that economic message and pulled him back into an extended conversation about contraception and other social issues.

Those issues did not play as well in states like Ohio, Michigan, Wisconsin and Illinois, where Mr. Santorum lost to Mr. Romney. Those losses helped create a mounting sense of frustration inside the Republican establishment that Mr. Santorum was waging a quixotic battle against Mr. Romney that would ultimately hurt the party’s chances against Mr. Obama.

Mr. Santorum’s decision will all but clear the way for Mr. Romney to claim the nomination.

Mr. Gingrich recently scaled back his ambitions, acknowledging that it was impossible for him to accumulate enough delegates to win the nomination before the national convention. Mr. Gingrich conceded on Sunday that Mr. Romney would most likely be the nominee, and said his primary goal in continuing to campaign was to influence the party’s platform at the convention.

Mr. Gingrich released a statement praising Mr. Santorum and his campaign, but also stressing his commitment to remaining in the race.

“I am committed to staying in this race all the way to Tampa so that the conservative movement has a real choice,” the statement read. “I humbly ask Senator Santorum’s supporters to visit Newt.org to review my conservative record and join us as we bring these values to Tampa. We know well that only a conservative can protect life, defend the Constitution, restore jobs and growth and return to a balanced budget.”

Mr. Paul also is continuing to wage his effort to win the Republican nomination and is scheduled to campaign this week.

But Mr. Santorum had been the last remaining candidate with a potential shot at stopping Mr. Romney — and even that opportunity was dwindling fast as Mr. Romney accumulated delegates.

Officials at the Republican National Committee had already begun considering the possibilities for beginning the traditional effort to merge their general election efforts with Mr. Romney’s Boston-based campaign. The decision by Mr. Santorum will make that easier.

It also could clear the way for Mr. Santorum to play a bigger role — and have a potentially bigger voice — in Mr. Romney’s campaign and perhaps in a Romney administration.

Katharine Q. Seelye and Jeff Zeleny contributed reporting.