DES MOINES, Iowa — Mitt Romney sought to convert his tentative standing atop the polls into a first-place finish in the caucuses here, telling Iowans on Sunday that he had the “capability to go the full distance” against President Barack Obama, as his rivals beseeched voters not to settle on a candidate lacking full commitment to their conservative values.

Just as confidence had been rising among Romney and his aides that they could pull off a win here Tuesday night, they were faced with a fresh challenge from Rick Santorum, who emerged as the latest in a rotating cast of surging alternatives, ebullient about his rising standing in the polls and support from excited crowds on Sunday in Sioux City and Rock Rapids.

“Don’t put forward somebody who isn’t good enough to do what’s necessary to change this country,” Santorum, the former Pennsylvania senator, declared at a town hall-style meeting in Sioux City, feeding off his new status as a real contender here. “Put forward someone that you know has the vision, the trust, the authenticity, the background, the record, to make that happen.”

Still decidedly in the mix was Ron Paul, the libertarian-leaning congressman from Texas, whose dedicated followers could still propel him into the lead Tuesday night and in the nominating contests that will unfold in the coming months.

Even though the Republican race remained fluid, the Democratic Party stepped up its involvement in the opposing contest, and several aides to the president’s re-election team arrived here to open a war room at a downtown hotel. The prime target was Romney. They introduced an Indiana worker who was laid off in the early 1990s when his company was restructured under the direction of Romney’s investment firm.

Iowa’s caucuses do not have an especially good record of predicting Republican nominees. But the result here could be an indicator of whether Romney, a former governor of Massachusetts, is succeeding in rallying conservatives behind him or whether he faces months of struggle to win delegates and resolve rifts within the party. After months of campaigning, a long series of debates and the rise and fall of one challenger after another, no one has yet shown that they can knock off Romney. But Romney, despite running a largely mistake-free campaign, has yet to prove that he can break through the ceiling of support of about 25 percent in national polls that has defined his candidacy in a fractured field.

Romney’s campaign aides were watching Santorum’s new strength carefully. They said that while they were satisfied that Santorum’s rise was further fracturing the anti-Romney vote between him, Paul, former House Speaker Newt Gingrich and Gov. Rick Perry of Texas, they could take nothing for granted when only half of likely Iowa caucusgoers say they have committed to a candidate.

And, on a day when all but the most politically involved Iowans were at home celebrating the new year and watching football, Romney’s campaign workers were calling the homes of potentially supportive caucusgoers they have been recruiting for months, wishing a happy new year to their families along with a gentle reminder to attend the caucuses.

Romney’s campaign had been optimistic enough about a possible victory here that it decided over the weekend to keep him in Iowa through Tuesday night to be in place for nationally televised interviews from Des Moines on Wednesday morning — a sign that they expected him to be talking about good news here.

But a senior aide, speaking on the condition of anonymity, played down the importance of a first-place showing, saying “our strategy was never based on a win in Iowa” and that the campaign would be “happily surprised” if he were to secure one.

Preparing for a potentially longer fight, Romney’s strategists in Boston were increasingly turning their focus on New Hampshire, South Carolina and, the biggest January primary state, Florida, where voters are receiving absentee ballots this week. The campaign has been aggressively working to get out the early vote there and his advertising team has begun to inquire about advertising rates across the state.

Sen. Charles Grassley, R-Iowa, said the competing messages in the campaign had left economic and social conservative activists “terribly split here.” Most campaigns were in agreement that a win by Romney would put him in an enviable position to capture his party’s nomination, as he has predicted he would this week.

None of the campaigns are prepared to count out a strong showing from Paul, who remains a wild card with top-tier — if eroding — poll numbers and a large war chest.

Less certain is the long-term strength of Santorum, who has struggled to raise money much of this year and ran his first television commercial only at the very end of the campaign here. But nothing fills a bank account like the word “surge” in headlines, and Santorum’s communications director, Hogan Gidley, said Sunday that his daily donations have increased 300 percent in the past few days. He said the campaign has bought advertising time in New Hampshire and is “working to put up an ad buy in South Carolina, as well.”