Gingrich, who won only his home state of Georgia in Tuesday's balloting across 10 states, said he's pressing ahead and will "wait and see how the race goes." He headed for Alabama and Mississippi, hoping to pick up more southern delegates next week.
Super Tuesday gave Romney a narrow victory in pivotal Ohio and wins in five other states, while Santorum laid claim to three states. Rep. Ron Paul won none. The split decision refreshed questions about Romney's appeal to conservatives, and guaranteed more convulsion ahead as Republicans struggle to settle on a candidate to take on President Barack Obama.
Leaders of a super political action committee supporting Santorum said it's time for Gingrich to step aside and let Santorum go head-to-head with Romney.
If Gingrich remains in the race "it's only a hindrance to a conservative alternative to Romney," said Stuart Roy, an adviser to the Red, White and Blue Fund. "And Romney simply won't be the conservative alternative to Obama."
The PAC has spent about $3 million on TV ads helping Santorum's White House bid, and Roy predicted that Wednesday would be "a good day for fundraising."
Santorum, who was campaigning Wednesday in Kansas and Mississippi, pointed to his wins in the West, the Midwest and the South as proof he can win across this country.
Romney, in an appearance on CNBC's "Squawk Box," insisted he's "getting the kind of support across the party that I need to become the nominee."
"We've got the time and the resources and a plan to get all the delegates, and we think that will get done before the convention," Romney said.
His campaign announced that Romney raised $11.5 million in February, the second-best month ever for the campaign. Still, that's not substantially ahead of Santorum, who raised $9 million in February.
Gingrich, in a morning appearance on Bill Bennett's "Morning in America" radio program, said there's no evidence Santorum could defeat Romney even in a one-on-one competition.
"If I thought he was a slam dunk to beat Romney and to beat Obama, I would really consider getting out," Gingrich said. "I don't."
Campaigning in Montgomery, Ala., Gingrich showed he had no intention of bowing out to clear a path for Santorum. He portrayed Romney as a moderate in the mold of losing GOP candidates Bob Dole and John McCain. And he cast Santorum as a creature of Washington, willing to play along with the team.
"There is a big difference between being a good team member and changing the game," Gingrich said. "I am not going to Washington to be a good team member. I'm going to Washington to change Washington."
Gingrich spokesman R.C. Hammond said Alabama and Mississippi, which vote on March 13, are must-win states for the former House speaker, although he stopped short of saying Gingrich would get out of the race if he lost there. To that end, Gingrich dropped plans to campaign Friday in Kansas, which holds caucuses Saturday, to focus on the southern states.
Surrogates made the case that it's time to close ranks around Romney.
House Republican Leader Eric Cantor of Virginia argued that Romney's claim to the nomination is inevitable, adding that Santorum and Gingrich "have not demonstrated an ability to do what needs to be done." But in an interview on CBS "This Morning," Cantor acknowledged there is still plenty of debate in "a robust party with many ideas."
Former New York Gov. George Pataki endorsed Romney in an appearance on Fox News Channel, saying it was time for Republicans to stop sniping at one another and focus on Obama.
"We are still bogged down shooting at each other and the president is looking presidential," he said.
Obama campaign manager Jim Messina, in a conference call with reporters, said Romney was "limping across the finish line" in primary states. He added that the weakness of the GOP field was giving the president an "expanded map" of states to work with in the fall campaign.
Vice President Joe Biden next week will begin a series of speeches laying out themes for Obama's re-election campaign, beginning in Ohio. The Obama campaign also plans to release a 17-minute documentary on Obama's first term by director Davis Guggenheim, whose credits include the Academy Award-winning "An Inconvenient Truth," about Al Gore's global-warming campaign.
In addition to claiming Ohio, Romney scored a home-state win in Massachusetts, and triumphs in Idaho, Vermont, Alaska, and Virginia. Santorum laid claim to Oklahoma, Tennessee and North Dakota.
Gingrich's win in Georgia, which he represented for several terms in Congress, was his first victory since he captured the South Carolina primary on Jan. 21.
Paul, the veteran congressman from Texas, had pinned his hopes on winning Idaho and Alaska but fell short in both.
Ohio was the marquee matchup, and for good reason. No Republican has won the presidency without carrying the state in the general election. With 99 percent of precincts reporting, Romney had 38 percent to Santorum's 37 percent, an uncomfortably close margin for a candidate who had spent nearly four times as much money in the state as his rival.
In all, 419 delegates were at stake across the 10 states: Romney picked up at least 212; Santorum got 84, Gingrich 72 and Paul at least 22.
That gave the former Massachusetts governor 415, more than his three rivals combined. Santorum was second with 176, Gingrich had 105 and Paul had 47. It takes 1,144 delegates to win the nomination.
In Oklahoma, Democratic officials were reviewing party rules to determine if the president lost a delegate to anti-abortion activist Randall Terry, who got 18 percent of the vote in the Democratic primary. Obama got 57 of the Oklahoma vote, and the rest of the state's vote was fractured. Under party rules, Terry is eligible for a delegate since he got more than 15 percent of the statewide vote.
Until Tuesday, Obama had won all Democratic delegates awarded so far.