Romney has been counting on a multi-candidate field splintering that vote to allow him to win, largely with the 25 percent of voters who backed him in 2008. Romney's central argument as he looks to win in Iowa: that he's the Republican best able to beat President Barack Obama.
Santorum, an anti-abortion crusader who is rising in polls at just the right time, is looking to unify socially conservative voters behind his candidacy. Their support has been splintered among several candidates until now. Romney's had trouble winning them over because of his Mormon faith and past switches on abortion and other social issues.
The issue of what type of candidate to choose cuts to the heart of why the Iowa race is so volatile; an NBC/Marist poll last week showed nearly even percentages of Iowa caucus-goers want a candidate who shares their values as want a candidate who can beat Obama.
"The first thing you see when you talk to any Iowa Republican is that desire to beat Barack Obama," Iowa GOP Chairman Matt Strawn said.
That's hurt Paul, who's looking to stem a recent slide as he's been attacked as out of the mainstream on foreign policy. The libertarian-leaning Texas congressman is trying to counter the suggestion that he's a fringe candidate, calling himself "electable" in a Sunday interview with ABC from his home state.
Texas Gov. Rick Perry, former House Speaker Newt Gingrich and Minnesota Rep. Michele Bachmann, all trailing, were looking to make final pitches in campaign stops Monday.
All are hoping they can sway the roughly half of likely caucus-goers who say they are undecided or willing to change their minds two days before the leadoff presidential caucuses.
Showing confidence - but facing pressure to perform - Romney planned a series of Monday rallies in cities across the eastern part of the state, areas he won in 2008.
While many Romney's events earlier this week have been held in small restaurants or coffee shops, Romney will kick off his Monday events at the Mississippi Valley Fairgrounds here. He also plans rallies aimed at earning local media coverage in Dubuque, Cedar Rapids and Des Moines.
On Sunday, Romney engaged with Santorum for the first time, offering mild criticism. Santorum "has spent his career in the government in Washington," Romney said during a campaign stop in Atlantic.
Santorum, meanwhile, was trying to counter Romney's central electability argument.
"I'm the candidate that actually was able to win in states, as a conservative, in getting Democrats and independents to vote for us," Santorum, the former Pennsylvania senator who is surging in the race and is a favorite among cultural conservatives, said in an interview on CNN. "Mitt Romney has no track history of doing that."
Gingrich, meanwhile, turned his fire on Romney. The former Massachusetts governor's allies have hit Gingrich with millions in negative TV ads in the past few weeks, helping to drive a significant slide.
"I feel Romney-boated," Gingrich said, referring to the outside group Swift Boat Veterans for Truth, which helped sink Democrat John Kerry's presidential bid in 2004. Gingrich planned a series of campaign stops in eastern Iowa Monday.
Gingrich, Perry and Bachmann were struggling to win support in the final days. A Des Moines Register poll released Saturday showed Romney and Paul locked in a close race, with Santorum rising swiftly to challenge them.
Nearly half of likely Iowa caucus-goers view Romney as the Republican most likely to win the general election. On that question, he was far ahead of Santorum and of Paul, who was viewed as the least likely to win.
This program aired on January 2, 2012. The audio for this program is not available.