Rick Santorum had been the last best hope of Christian conservatives who opposed Mitt Romney, derided by many as a “Massachusetts moderate.” Now, facing the abrupt end of Santorum’s presidential bid, some evangelical leaders have begun to rally behind Romney, saying their shared hostility toward President Barack Obama will be a powerful force for uniting the party in November.

In one sign of coalescing support from Christian conservatives, the National Organization for Marriage, a leading opponent of same-sex marriage, endorsed Romney on Wednesday morning. The group called Romney a “true champion” and said that Obama “has done virtually everything in his power to undermine the institution of marriage.”

But the whole-hearted support of evangelicals, who accounted for nearly one-fourth of all ballots cast in recent presidential elections, will not come without conditions, some leaders warned. During the bitterly fought primary campaign, many conservatives questioned the depth of Romney’s opposition to abortion, same-sex marriage and government spending. They say that to win the presidency, Romney may need a fired-up base to produce a large evangelical turnout in swing states like Missouri, North Carolina, Ohio and Virginia.

If Romney is to generate more excitement and sacrifice from Christian conservatives, he must “demonstrate a genuine and solid commitment to the core values issues,” said Tony Perkins, president of the Family Research Council. Perkins said Santorum had vaulted into conservative favor because “he passionately articulated the connection between America’s financial greatness and its moral and cultural wholeness” and recognized that “the economy and the family are indivisible.”

“Intensity is going to be a big problem for Mitt Romney,” Perkins said in an interview, comparing an obligatory vote for Romney, to avoid the alternative of Obama, to “eating your vegetables.”

Romney, a range of evangelical leaders said, must not waver in his support for conservative principles if he hopes to tap the energies of Christian conservatives in the general campaign.

But he also needs independent votes to win, said Whit Ayres, a Republican strategist and founder of North Star Opinion Research in Alexandria, Va. So Romney must deftly walk a political tightrope, keeping the focus on economic issues with broad appeal, Ayres said.

Romney’s aides are exasperated by the skepticism, because he has formally checked the right conservative boxes, pledging opposition to same-sex marriage, strong support for Israel and the military budget, and embracing the stringent fiscal proposals of Rep. Paul D. Ryan, R-Wis., a Tea Party favorite. But his inconsistent history on social issues, in particular, leads some to question whether he would carry through as president.

Gary L. Bauer, the president of American Values, a Christian advocacy group, had been one of Santorum’s most visible supporters because, he said in an interview, Santorum best personified the Reagan legacy of fiscal and social conservatism and a strong defense. Still, he is not reluctant to shift his allegiance now, though with a warning.

“Going to the general election, I will do everything I can for Gov. Romney,” he said.

“But his campaign has got to make it easy for me to help them,” he added, “and not make it hard by being tempted to pull back on conservative issues.”

Evangelicals were excited last August when Rick Perry, the Texas governor, entered the race, but he soon fizzled after poor debate performances. More than 100 Christian leaders met last January on a Texas ranch to seek a consensus candidate. Most cast their lot with Santorum, but some stayed loyal to Newt Gingrich, resulting in a divided evangelical vote in subsequent primaries.

Now, many evangelical leaders reject the conventional wisdom that Romney, after steering to the right to gain the nomination, must tack toward the center to win in November. They point to John McCain’s loss in 2008, when social conservatives were lukewarm about his candidacy and voted in modest numbers, and argue that in many important states, Republicans will need a higher evangelical turnout, like that seen in the 2010 congressional elections, to win.

Conservatives will be scrutinizing Romney’s every word and gesture in the months ahead, especially signals like his choice of vice-presidential candidate — they mention Sen. Marco Rubio of Florida, Ryan, Sen. Rob Portman of Ohio or even Mike Huckabee, the former Arkansas governor and Baptist pastor, as reassuring choices.

Romney’s own campaign officials and conservative supporters reject any suggestion of altering course for the general election. They argue that Romney’s positions on core issues are solid and that Romney does not face a serious dilemma as he tries to attract both conservatives and independent voters. By stressing the overriding issues of the economy and debt and Obama’s economic failures, in this view, he can appeal to both groups at once.

Bauer also noted that in parts of the country, independent voters include many blue collar workers who are most concerned about jobs but also share conservative values on abortion and marriage.

Peter Flaherty, a senior advisor to the Romney campaign, said in an interview Tuesday that if Romney secured the nomination, he would reach out to conservatives who had opposed him, nurturing ties. “We are going to sit down with everybody involved with the other candidates’ campaigns,” he said.

Jay Sekulow, chief counsel for the American Center for Law and Justice, a Christian legal group, and a supporter of Romney, said the “stark contrast” between Romney and Obama in their visions of America would energize evangelicals as well as attract independent voters.

“I think Gov. Romney just needs to stay true to himself, to continue articulating his positions on the economy and the Middle East,” Sekulow said.

But some conservative Christian leaders, even as they say they will of course vote for the Republican candidate, remain guarded about Romney. Steve Scheffler, president of the Iowa Faith and Freedom Coalition, complained that in Iowa, where Santorum’s surprise showing in the January caucuses thrust him into the limelight, Romney’s campaign had so far made no effort to reach out to evangelicals.

The presidential race is likely to be truly competitive, Scheffler said, in about a dozen states, including Iowa, and the Republicans will need to motivate people to volunteer at calling centers and put up signs. “I don’t think there’s much room for error,” he said, “and for them not to be having these conversations with us has been baffling.”

“I hope it changes, because they’re going to need Iowa,” Scheffler said.

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