By Sam Youngman and Steve Holland
BOSTON (Reuters) - Republican presidential hopeful Mitt Romney's campaign calls rival Newt Gingrich a formidable opponent, but aides to the former Massachusetts governor believe their better organized and well-financed organization can outlast Gingrich.
Romney has been at the top of the Republican field for most of the year, often maintaining a steady if underwhelming lead.
However, after strong gains in recent weeks Gingrich has shot to the top of polls in the critical early states of Iowa and South Carolina and closed Romney's lead in New Hampshire with voting to start in weeks.
In response, Romney has unleashed two surrogates, former Republican senators Jim Talent of Missouri and John Sununu of New Hampshire, and begun running TV ads to try to raise doubts about the former speaker of the House of Representatives.
A senior campaign official said on Thursday that Romney is prepared for a nomination fight that could go into June. His campaign believes Romney's organization and more time for voters to re-examine Gingrich's record will play to Romney's ultimate advantage.
The official said the Romney camp is focused on winning the Republican presidential nomination to challenge President Barack Obama in 2012 and "whether it takes us 30 days or 90 days or more, that's what we have to do."
The Romney campaign believes the election process, which awards nominating delegates on a proportional basis rather than winner-take-all, will play to Romney's advantage and that the former Massachusetts governor will have enough money to compete in all states in primary battles that extend into the summer.
The Romney official insisted that the campaign was not caught off-guard by Gingrich's resurgence after his candidacy was given up for dead last summer when much of his staff quit over the management of the campaign.
Gingrich is the latest in a series of anti-Romney conservative alternatives like Texas Governor Rick Perry and businessman Herman Cain to have a moment in the sun. The belief in the Romney camp is that his star will fade like the others, although many political analysts see the Gingrich surge as far more formidable than the others.
"Almost everyone has had a moment," the official said. "It's not surprising that Newt would have a moment."
The campaign official described Gingrich as "more formidable Herman Cain," the former pizza magnate who dropped out of the race last week.
But the official conceded that Romney will not wait for the thrice-married Gingrich to implode on his own as Cain did under the weight of charges of sexual harassment and an extramarital affair.
"You can't count on that," the official said. " That would be a bad strategy."
Despite the shockwaves Gingrich's ascension has sent through Washington, the Romney official insisted that Gingrich's surge has caused the campaign to alter its strategy only slightly. In Iowa, for example, Romney bought television ads only three days earlier than he had originally planned to.
On Thursday morning, hours after new polls showed Gingrich seizing commanding leads in Iowa and South Carolina, Romney unleashed Talent and Sununu to go after Gingrich for his criticism of a budget plan that was formulated by conservative Paul Ryan, chairman of the House Budget Committee.
Campaign officials said voters can expect to see more of that, from Sununu, Talent and others, as the campaign goes on.
(Editing by Christopher Wilson)