By Sam Youngman and Jeff Mason
NORTH LIBERTY/MASON CITY, Iowa (Reuters) - Mitt Romney's campaign did its best on Wednesday to lower expectations, but no one was really buying it.
As six of the contenders for the Republican nomination for president barnstormed across Iowa, there was a clear sense that Romney could be in position to lock down the nomination far sooner than could have been expected just a few days ago.
Urging Iowans to vote in the caucuses that kick off the nominating process next Tuesday, Romney and his aides were surprised at the size and enthusiasm of overflow crowds at diners and coffee shops.
And two new polls -- one from Public Policy Polling showing the Massachusetts governor running a close second to Texas congressman Ron Paul; another from CNN showing Romney ahead -- suggested that the dynamics of the race were shifting in Romney's favor.
In contested races under the current primary system, it has been rare for a nonincumbent to win the Republican nomination after winning both the Iowa caucuses and the New Hampshire primary that follows, this year on January 10.
Romney, who has a big lead in New Hampshire polls, could pull off that feat and gain a strong grip on the nomination -- if he can win Iowa.
On Wednesday Romney was having a hard time containing a smile at the thought of striking such a huge blow so early in the primary season.
"I can't possibly allow myself to think in such optimistic terms," Romney said during a stop at Homer's Bakery in Clinton, Iowa.
The polls also indicated that former House speaker Newt Gingrich of Georgia -- who led in the polls in early December -- could fade out of contention before he gets to the January 10 primary in South Carolina, where he still leads and has put much of his hope.
In Iowa, Gingrich was third in the Public Policy survey and fourth -- behind former Pennsylvania senator Rick Santorum -- in the CNN poll.
As his standing in the polls has fallen amid a barrage of negative TV ads from Romney, Paul and a group supporting Romney, Gingrich has been pointing to South Carolina, where he is building up his staff.
Gingrich, whose modest campaign was traveling around Iowa in a car just a few weeks ago, is now traversing the state in a giant bus with "NEWT 2012" emblazoned on the side.
Although he declared himself the front-runner in the race in early December after strong debate performances shot him to the top of voter surveys, the Gingrich campaign now is considerably more humble.
He told CNN this week that he would be fine if he finished in the "top three or four" in Iowa, but political strategists said that type of showing Tuesday could damage his campaign.
"If he wants to be one of the two finalists, he has to win Iowa," said Charlie Black, a top adviser to John McCain's 2008 presidential campaign who is backing Romney. "Unless he wins there, he's going to have a very hard time fighting his way back into the race."
Republican strategist Ron Bonjean, who has not backed a candidate, said Gingrich's attempts to lower expectations were not going well.
"His efforts to manage expectations are proving difficult after he pledged to win the state," Bonjean said, adding that Gingrich should focus on placing in the top three in the January 10 primary in New Hampshire.
Then, Bonjean said, Gingrich could seek to shore up his leads in the next two primary states, South Carolina and Florida, which are closer to Gingrich's conservative southern base.
FOR ROMNEY, A PLEASANT SURPRISE
The notion that Romney could come roaring out of Iowa with a win has seemed preposterous for much of the campaign.
As a candidate in 2008, Romney invested heavily in Iowa. But Iowa's Christian conservatives became infatuated with former Arkansas governor Mike Huckabee, a Baptist preacher.
Romney finished second in Iowa, then lost New Hampshire to eventual nominee John McCain, the Arizona senator.
For much of 2011, Romney did not seem particularly interested in campaigning in Iowa.
But when conservative Texas Governor Rick Perry stumbled in debates, Romney's team began pouring resources into Iowa -- and spending big on TV ads.
Perhaps the most devastating ads against Gingrich were put on the air by a political action committee that supports Romney. They cast Gingrich as a Washington insider who earned millions as a lobbyist after leaving the House.
Gingrich has called the ads unfair.
At a shopping mall Wednesday in Mason City, Gingrich said that "the only person helped by negative ads is Barack Obama, and our business is to defeat him, not to help him."
Gingrich then told a small crowd, "I would ask you, when you go to the caucus, to remind your friends and neighbors: Do you really want to reward somebody who's done nothing but run negative ads? Or do you want to elect somebody who has big, positive solutions, runs positive ads, and has a track record of actually getting it done?"
When asked Wednesday whether he will win the state, Romney demurred, saying that both Gingrich and Paul have led polls in the state.
Romney added that the race for the nomination won't be over until a candidate is backed by 1,150 delegates, which are awarded proportionately by state. Twenty-eight delegates are at stake in Iowa.
"I like the fact that my support is building and the momentum is positive, but I can't tell" who will win, Romney said. "I'll tell you this: I've got to get 1,150 delegates. And I'd like to get a good start here and in New Hampshire and in South Carolina and Florida."
Romney supporter Indra Brewer wasn't so reserved.
"I think he can" win Iowa, she said. "The momentum's building, and I think that's going to pay off for him in the end."
Gingrich supporters, meanwhile, are holding out hope.
Iowan Jerry O'Neill, 72, said he was glad Gingrich was staying positive.
"He's dropping in the polls, but polls fluctuate," O'Neill said. "It's coming down to the wire as far as the Iowa caucus, but it's not the end of the road."
(Editing by David Lindsey and Cynthia Osterman)