DES MOINES, Iowa — Unpredictable to the end, many of Iowa’s GOP voters still hadn’t settled on a favorite candidate just hours before they cast the first ballots of the 2012 presidential contest.

After weeks of face-to-face campaigning and millions of dollars in advertising, Republican presidential hopefuls made last-minute appeals to undecided GOP voters.

A confident Mitt Romney, the front-runner for the GOP nomination, told a rally Monday, “We’re going to win this thing.”

Whether he would pull that off was far from clear.

On Tuesday night, Republicans will gather in living rooms, high school gymnasiums and local libraries to start the process of picking the 2012 GOP nominee. In each precinct caucus, voters will urge their friends and neighbors to support a preferred candidate.

The two who appeared most likely to challenge Romney for the Iowa victory were former Pennsylvania Sen. Rick Santorum and libertarian Rep. Ron Paul of Texas. Most polls in recent days have put Romney and Paul atop the field in Iowa, with Santorum in third and gaining ground. More than a third of all potential caucus-goers said they could yet change their minds.

Texas Gov. Rick Perry, former House Speaker Newt Gingrich and Minnesota Rep. Michele Bachmann all trailed. Gingrich on Monday went so far as to predict his own defeat, and Paul said he does not envision himself in the White House.

Neither Santorum nor Paul is likely to present as serious a challenge to Romney as would Perry and Gingrich, who have both fallen back.

“Do not settle for less than what America needs to transform this country. Moderate candidates who try to appeal to moderates end up losing,” Santorum said Monday in a slap at Romney.

Romney faces the same challenge he did in 2008: winning over a conservative base that’s uncomfortable with his moderate past. In 2008, socially conservative voters united behind Arkansas Gov. Mike Huckabee, denying Romney a first-place finish and contributing to his eventual defeat.

This time, Romney’s trying to win Iowa by arguing he’s the most electable candidate against Democratic President Barack Obama — a pitch that’s winning over conservatives who desperately want to beat the president.

“I want to make sure I vote for and caucus for someone who is a winner. We cannot have another four years of Obama,” said eyeglass salesman Paul Massey, 65.

“It might come down to the speeches at the caucuses. I want to support someone who can go all the way and defeat the Democrats in November,” Phil Ubben of Sioux City said.

How many people turn out to vote will help drive the results. In 2008, more than 120,000 Republicans showed up, a record. Weather could be a factor in this year’s attendance. Iowa hasn’t had much snow this winter, and there were clear but cold forecasts across the state.

After Tuesday’s vote, Romney, Gingrich and Santorum planned to depart immediately for New Hampshire. Romney holds a commanding lead in polls there, and will be in a strong position to win even if he doesn’t pull out a victory in Iowa. Paul plans to join his rivals in New Hampshire later in the week. The primary is Jan. 10.

Perry and Bachmann don’t plan to compete in New Hampshire, instead heading straight from Iowa to the first-in-the-South primary, set for Jan. 21 in South Carolina. Romney also plans to visit South Carolina this week, with campaign stops Thursday and Friday.