Romney had 573 delegates per the Republican National Committee's count to Santorum's 202 delegates when the former U.S. senator from Pennsylvania withdrew last week. But he has yet to stir a fire in the belly of the party's evangelical and conservative wings as he moves closer to the 1,144 delegates needed to claim the nomination.
"Some of Santorum's delegates will immediately shift to Romney, some will gradually shift, and a hard core group will probably go to another candidate or remain uncommitted," political commentator Steven Schier of Carleton College in Northfield, Minn., said. "This last group will probably not be large enough to disrupt a Romney coronation at the convention."
The final Gallup poll conducted during the party preference process, released last week, indicated Romney is the preferred presidential candidate of 42 percent of Republicans polled nationwide, among the lowest measured in a final primary poll since 1972, Gallup said. Santorum was the choice of 24 percent of Republicans in the poll completed before he made his announcement, the Princeton, N.J., pollster said.
Even though he trailed -- badly -- in money and delegates, Santorum's strong performance created a sometimes bruising primary campaign. Santorum won 11 states before he bowed out ahead of his home state's primary April 24 where several polls indicated Santorum held a slim single-digit lead over Romney and one survey showed Romney leading Santorum.
Santorum's withdrawal cleared the way for Republicans to rally around Romney, and three prominent conservatives, Govs. Bobby Jindal of Louisiana and Rick Scott of Florida, and Sen. Patrick J. Toomey of Pennsylvania endorsed Romney hours after Santorum announced his departure.
Conservative leaders who had rallied around Santorum's campaign, however, have raised doubts that faction of the Republican Party could readily embrace Romney, seen by that wing as too moderate, The New York Times reported.
Conservative activist Richard Viguerie was blunt, especially since Romney unleashed a barrage against Santorum and other conservative candidates.
"After having destroyed every conservative that came on the scene," Viguerie said, "you can't say 'You have to line up behind me.' No, no, no. Conservatives are not going to jump until they hear where Governor Romney wants to take everybody."
"I just think it's going to be a much harder lift to take someone who seems like a moderate and try to get conservatives excited about it," Family Research Council President Tony Perkins told CNN.
Perkins said Romney shouldn't expect to collect the same type of support Santorum got if the former Massachusetts governor doesn't fully embrace the principles of the socially conservative organization.
"And so if the party is moving in a different direction, we are not going there," Perkins said. "The only reason there was an alignment with Rick Santorum from our constituency is because Rick embraced the ideas, the policies and the principles that our organization and our constituency believes in and so to the degree that one candidate or another aligns with that, they're going to find support. If they don't, they're not going to get the unbridled enthusiastic support that Rick Santorum enjoys."
Even though Santorum's departure removes the last major stumbling block to Romney's nomination two other candidates -- former House Speaker Newt Gingrich and U.S. Rep. Ron Paul of Texas -- said they would stay in the race until the Republican National Convention in Tampa, Fla., with each claiming he represents the conservative alternative.
Without flat-out saying Gingrich and Paul should end their bids, Republican National Committee Chairman Reince Priebus told CNN, "I think they should all look at the math."
"Gingrich and Paul may remain on the stump, but they will receive relatively limited media attention because the nomination contest is essentially over," Schier said. "Romney's challenge regarding these candidates is obtaining their strong personal support by the convention so that he can launch the fall campaign without a divided party."
Gingrich and Paul both have personal agendas they want to bring to the fore, Schier said.
"For Paul, it is libertarianism. For Gingrich, it is strong social and fiscal conservatism," the commentator said. "For these reasons, they remain at least nominally active candidates. It would be best for Romney if both candidates dropped out and endorsed him, but neither is likely to create considerable disruption at the convention."
Romney and Santorum traded vitriol throughout the campaign, but former Minnesota Gov. Tim Pawlenty, a leader on Romney's campaign team, said he expected Santorum to rally behind Romney.
"[We] need all of the pieces to come together to make this a successful campaign," said Pawlenty, who abandoned his bid to be the GOP nominee after the Iowa straw poll last summer. "But ... there's a grand tradition of people competing hard for party nominations" then uniting against a common foe.
Priebus said told CNN he believed Republicans would be "100 percent unified behind our nominee."
"I think time heals some wounds," the RNC leader said. "And I also think that over time, over the next several weeks, I think our party gets completely unified."
The main challenge for Romney within the GOP is "generating enthusiasm among the evangelicals and strong conservatives who have thus far disdained him," Schier said. "He needs their strong support in the form of volunteer hours and campaign contributions. He will be conducting an outreach over the next several months to secure that support because he needs their support to be competitive with President Obama this fall."