With the first-in-the-nation voting of the 2012 race for the White House looming Tuesday, Mitt Romney is contending for victory in a state that eluded him four years ago, while Rick Santorum - a hero among social conservatives - surges and libertarian-leaning Ron Paul slides in a contest that remains incredibly fluid.
With many factors at play, the dynamics can shift rapidly.
Yet, two things were clear on the final weekend before the caucuses: The yearlong effort to establish a consensus challenger to Romney had so far come up short, and Romney's carefully laid plan to survive Iowa may succeed because conservative voters have so far failed to unite behind one candidate.
"It may be Romney's to lose at this point," said John Stineman, an Iowa GOP campaign strategist. "And it's a battle among the rest."
Underscoring the unpredictability of the race, a new poll by The Des Moines Register showed that a remarkable 41 percent of likely caucus-goers say they were undecided or still might change their minds.
Romney had 24 percent support among likely voters while Paul had 22 percent, meaning they were statistically even at the front of the pack. Santorum was third with 15 percent, followed by Newt Gingrich, with 12 percent, and Rick Perry, with 11 percent. Michele Bachmann, a one-time Iowa favorite, brought up the rear with 7 percent.
However, in a sign of how quickly things can change, the last two days of the poll - taken Tuesday through Friday - found Santorum with momentum and Paul losing his. Heading into the weekend, Romney held a narrow lead, but Santorum was right behind him with 21 percent while Paul had fallen to 18 percent.
The poll's margin of sampling error for the full four days was plus or minus 4 percentage points. For the last two days, it was plus or minus 5.6 percentage points.
On Sunday, the candidates were making their closing arguments, both in appearances across Iowa as well as on national television, while volunteers and staffers canvassed the state to start mobilizing backers.
Paul, who was at home in Texas for the weekend, was making the rounds of Sunday talk shows, while Santorum, Perry and Bachmann were doing the same from Iowa.
Interviewed on CNN's "State of the Union," Paul discounted the impact of Santorum's surge on his own campaign.
"It's the people who got frustrated with the other ones and they're just shifting their views," he said. "That's one thing you can't say about my supporters. They don't shift their views."
Paul predicted a strong showing Tuesday, saying he would likely finish first or second.
Romney planned appearances in Atlantic and Council Bluffs as he works to maximize the edge he holds in critical areas rather than risk underperforming in places where more ardent conservatives are leery of his Mormon faith and shifting positions on social issues.
In Le Mars on Sunday, he drew a crowd of 300 people, including supporter Alan Lucken, who shouted to the candidate: "You're going to win."
"I'm planning on it," Romney said and later told a reporter, "I sure hope to. I'll tell you that."
In another show of confidence, Romney promised to return if he is the GOP nominee.
"I'm going to be back in Iowa; we're going to fight, we're going to win Iowa in the general election," Romney said as he closed his remarks in Le Mars.
Santorum, meanwhile, looked to capitalize on his recent surge by focusing on southern portions of rural Iowa, where the former Pennsylvania senator has made a point of visiting more often than his rivals. And his campaign rolled out a new TV ad casting him as "a trusted conservative who gives us the best chance to take back America."
He claimed momentum Saturday - and acknowledged his opponents had more money - as he traveled with his daughter Liz, who quit college to campaign for her father.
"We believe that ultimately, money doesn't matter in Iowa," Santorum said at a packed stop in Indianola. "You can't buy Iowa. You've got to go out and work for Iowa votes."
Perry's advisers see Santorum within reach and have begun attacking the former senator for having supported spending on home-state pet projects, an unpopular position in these tough economic times.
"I think the world of Rick Santorum. He's got a great family. But we've got some real difference when it comes to fiscal issues," Perry told supporters in Boone. "Those differences couldn't be clearer when it comes to important issues in this election like spending."
Santorum, in turn, charged Perry with hypocrisy: "He had a paid lobbyist in Washington looking for earmarks."
Perry announced he would travel directly from Iowa to Greenville, S.C., the day after the caucuses, bypassing next-up New Hampshire. Still, he said he planned to participate in two debates in New Hampshire next weekend.
Gingrich, for his part, was spending the weekend pleading anew with Iowans to side with him despite what they have learned about him through millions of dollars in attack advertising by Paul and a political action committee bankrolled by Romney supporters.
"Iowa could actually dramatically change people's understanding of what works in politics if you repudiate that kind of negativity," Gingrich told 150 people at a Council Bluffs restaurant.
Associated Press writers Holly Ramer in Campton, N.H., Steve People in Hampton, N.H., and Shannon McCaffrey in Council Bluffs and Brian Bakst in Urbandale contributed to this report.