Former House Speaker Newt Gingrich, with little to show for pinning his hopes on the South, nonetheless vowed to stay in. His deputy campaign manager outlined a strategy aimed at denying Romney a clean win before the Republican convention in August and making Gingrich's case to delegates along the way.
But after winning Tuesday's primaries in Alabama and Mississippi, Santorum called for the party's faithful to unite behind him alone.
"Now is the time to pull together," Santorum declared to conservatives in Lafayette, La. "We are campaigning everywhere there are delegates because we are going to win this nomination before the convention."
Unbowed, Romney issued a statement noting his strong lead in the delegate race, saying, "I am pleased that we will be increasing our delegate count in a very substantial way after tonight." Earlier Tuesday, in an interview with CNN, Romney had said Santorum was "at the desperate end of his campaign."
Romney fared better in the night's two Pacific island contests. He salvaged a win in the Hawaii caucuses and won the support of all nine delegates at GOP caucuses in American Samoa. That means Santorum's two latest wins gain him little or no ground in the delegate count, despite their symbolic weight coming after his victories last week in Tennessee and Oklahoma.
The race now turns to Missouri's caucuses Saturday and Puerto Rico, which is reaping unusual attention before its primary Sunday. Illinois follows on Tuesday and Louisiana on March 24.
Santorum flew to San Juan fresh off his Southern victories, and planned events over the next two days. Romney wasn't slated to arrive in the U.S. territory until the end of the week, after spending two days in New York City to raise money.
Romney's seemingly unassailable delegate lead left his opponents' campaigns talking about less orthodox ways to stop him. John Brabender, senior strategist for the Santorum campaign, said many of the delegates weren't bound and could still switch their votes to Santorum.
Suggesting it's time for Gingrich to make way, Brabender told CNN Wednesday morning that the message was going out to tea party and conservative voters: "Let's make sure our voice is louder than the minority of the party who wants Mitt Romney."
Tuesday night's results marked the continuation of a long, hard-fought Republican nomination fight — and underscored Romney's persistent weakness with conservatives, particularly in the GOP's regional stronghold of the Deep South. Together, Santorum and Gingrich accounted for huge majorities of votes in Alabama and Mississippi, prompting Gingrich to crow: "The fact is, in both states, the conservative candidates got nearly 70 percent of the vote. If you're the front-runner and you keep coming in third, you're not much of a front-runner."
Romney had been hoping for at least one Southern victory Tuesday that might have allowed him to start arguing it was time for the party to gather behind him and begin the general election fight against President Barack Obama.
Instead, Romney now faces a resurgent Santorum — and he is without the overwhelming financial advantage he boasted throughout the early states. Romney's campaign trimmed some spending in recent weeks as he was forced to spend more time campaigning and less time fundraising. Still, he's got more delegates than his rivals combined.
Santorum's victories Tuesday were worth at least 35 delegates, but Romney won at least 41. Gingrich won at least 24, while Ron Paul picked up at least one. The delegate split underscored the difficulty that Romney's rivals face in overcoming his big lead.
The partial allocation of delegates from Tuesday's voting states left Romney with 495 in The Associated Press count, out of the 1,144 needed to win the nomination. Santorum had 252, Gingrich 131 and Paul 48.
And while Santorum in particular challenges the mathematical projections, Romney is amassing delegates at a rate that puts him on track to clinch control of nomination before the convention next summer.
A senior Romney adviser, Jim Talent, said the campaign is where it needs to be. "We're really running against the delegate totals more than any of the others," Talent told CNN Wednesday.
Gingrich deputy campaign manager Vince Haley suggested the former House speaker was putting himself in position to compete at a brokered convention, saying Gingrich could "win a debate in this country over the course of the rest of this campaign."
It is rare for Alabama and Mississippi to play an important role in a Republican nominating campaign, but the 2012 race has gone on far longer than usual. Equally improbable was the decision by Santorum and Romney to spend time during the next few days in Puerto Rico.
The Romney-aligned super PAC is already advertising in Illinois in hopes of gaining an advantage there.
All three candidates, as well as the super PACs supporting each of them, ran television commercials in Tuesday's Southern states. As has been the case all year, Restore Our Future, which backs Romney, spent more than any of the others, putting down $1.3 million for television ads in Alabama and another $900,000 in Mississippi.
Associated Press writers Stephen Ohlemacher and Connie Cass in Washington contributed to this report.