Seeking to press their advantages and differentiate themselves, the Republican candidates for president flooded the Iowa airwaves Sunday and stepped up their ground games in the final push before decision day.

Some went to church, others were on buses and at least one — Rep. Ron Paul — was somewhere else entirely, ringing in the new year back in his home state of Texas.

Rick Santorum, buoyed by a Des Moines Register poll that showed him among the top three contenders, sought to distinguish himself as the only “full spectrum conservative” in the race; it was an effort to ding Mitt Romney and appeal to social conservatives, who could be the decisive voting bloc should they coalesce around a single candidate.

“My surge is going to come on January 3 after the people of Iowa do what they do, which is actually analyze the candidates, figure out where their positions are, find out who’s the right leader, who’s got what it takes to defeat Barack Obama and to lead this country,” the former U.S. senator from Pennsylvania said in an interview on “Meet the Press.” “We’ve got a great grass-roots organization. . . . They are committed to making sure that this isn't a pyrrhic victory.”

Santorum sought to paint Romney, whose essential argument for support is his electability, as a moderate who can’t be trusted to push a conservative agenda.

But Santorum had to answer questions about his 2008 endorsement of the former Massachusetts governor and about seeming to change his own views on abortion based on political calculus.

Santorum, who signed a pledge opposing all types of abortion, including in cases of rape and incest, said passing that kind of legislation is politically difficult. “Today I would support laws that would provide for those exceptions; but I’m not for them,” he said. “Yes, I support laws that provide those exceptions, because if we can get those passed, then we need to do that.”

Romney, on a bus tour in Atlantic, Iowa, reminded reporters of Santorum’s 2008 endorsement and pressed his case that he was the best candidate to beat President Obama on the economy.

“Like Speaker Gingrich, Senator Santorum has spent his career in government, in Washington,” Romney said. “Nothing wrong with that, but it’s a very different background than I have. And I think that the people of this country recognize that with our economy as the major issue we face right now, that it would be helpful to have someone who understands the economy firsthand, who spent the bulk of his career working in the private sector.”

With just hours left, the race remains topsy-turvy, with nearly half of voters saying that they have yet to make up their minds. The key questions are whether social conservatives will rally around a single candidate, as they did for former Arkansas governor Mike Huckabee in 2008, or continue to be splintered among several. There is also the possibility that they accept Romney’s electability argument and rally around him.

Santorum, in his ground campaign, has argued that he should be the candidate of social conservatives and that they should not just “settle” for Romney. And evangelicals, not yet sold on Romney, could decide to send a message Tuesday that they still matter and that the eventual nominee must take them seriously.

Former House speaker Newt Gingrich, battered by an onslaught of negative ads and somewhat hampered by his pledge to run a positive campaign, started his Sunday with Mass at St. Ambrose Cathedral, where Des Moines Bishop Richard E. Pates referenced the nasty campaign rhetoric flooding the airwaves.

After the service, Gingrich took a swipe at Romney, breaking sharply from his pledge.

Gingrich told reporters that Romney “would buy the election if he could” and accused Romney of lying. “Someone who will lie to you to get to be president,” Gingrich said, “will lie to you when they are president.” The former speaker also signaled that he intended to go negative on Romney with new ads in New Hampshire.

Paul skipped out on the action over the weekend, preferring to spend time in Texas. He makes his final rounds Monday in a series of events with Sen. Rand Paul (R-Ky.), his son, who is becoming a familiar face to Iowa voters and remains a tea party standard-bearer.

The 12-term congressman, who is in a virtual tie with Santorum and Romney in Iowa, sat for three Sunday show interviews. He has largely ducked news media questions on the trail as he has rallied hundreds of supporters with an antiwar message.

Dogged by questions about racist and anti-Semitic newsletters that were published in his name, Paul admitted to one flaw, saying his management style is substandard.

Asked to predict where he’ll stand Tuesday after the votes are tallied, Paul was his usual anti-politician self.

“I have no idea what’s going to happen. I may come in first, I may come in second,” he said. “I doubt I’ll come in third or fourth.”

Staff writers Amy Gardner and Philip Rucker contributed to this report.