I had written on Saturday night, before the results of the South Carolina primary were known, that Mitt Romney could not necessarily bank on holding down a lead in Florida despite recently having had a double-digit advantage in the polls there.

These polls had not yet accounted for the large swing against Mr. Romney and toward Mr. Gingrich in state and national polls. The Gallup national tracking poll, for instance, now shows Mr. Romney at 29 percent and Mr. Gingrich at 28 percent, which represents a 23-point swing from 8 days ago when Mr. Romney led Mr. Gingrich, 37-13. And the swing against Mr. Romney in South Carolina — from a lead of about 12 points in the polls early last week to a loss by about that margin on Saturday night — was closer to 25 points.

Two new automated polls suggest that the swing in momentum — plus whatever additional gains Mr. Gingrich made from the favorable news coverage following his South Carolina win — are enough to have given him the lead in Florida.

The first poll, from Rasmussen Reports, puts Mr. Gingrich ahead by 9 points, 41 to 32. The second poll, from Insider Advantage, pegs Mr. Gingrich’s lead at 8 points. Both polls conducted all their interviews on Sunday in the immediate aftermath of Mr. Gingrich’s South Carolina victory.

Polls from InsiderAdvantage have generally had more favorable results for Mr. Gingrich than those conducted by other polling firms, and the C.E.O. of the polling firm, Matt Towery, is a former Gingrich campaign chairman.

Rasmussen Reports polls, however — although they have sometimes shown a Republican “lean” in general elections — have not been particularly friendly to Mr. Gingrich in recent weeks. Instead, the firm’s previous poll of Florida — conducted just 11 days ago — had given Mr. Romney a 22-point lead there.

Although polling is often volatile in nomination races, it has not been common in the past to see swings of quite this large a magnitude. Notable exceptions would include John Kerry in 2004, who had a major surge in the polls both before and after his win in the Iowa caucuses, and Gary Hart in 1984, who overcame what had once been a 40-point deficit to Walter Mondale in the New Hampshire primary to win there.

Mr. Hart, however, lost the nomination to Mr. Mondale after a long delegate battle. In some ways, that race bears some resemblance to the Republican contest this year. Mr. Mondale, like Mr. Romney, had the overwhelming support of the party establishment, and he won in part because of his strong showing in caucus states, an advantage that Mr. Romney is also likely to have if he can keep his footing in Florida.

This Hart-Mondale precedent is not an ideal one for Mr. Romney because it contributed to very low Democratic base turnout in the general election in 1984.

Still, he might prefer it to some of the alternatives. The FiveThirtyEight forecast model, which keys heavily on a candidate’s momentum, now shows a potential double-digit win for Mr. Gingrich in Florida. This is a plausible outcome, and one that could put Mr. Romney’s campaign in grave jeopardy.

However, the model’s estimates are likely to be somewhat unstable until more polls are released in Florida. The momentum in this nomination race has sometimes reversed itself literally overnight, so I would advise against it taking its estimates too literally until we have more data to look at.

One hopeful sign for Mr. Romney is that he held an 11-point lead over Mr. Gingrich in the Rasmussen poll among Floridians who have already voted. News accounts have sometimes overstated the impact of early voting — about 225,000 Republicans have voted so far, but that would represent only about 12 percent of a turnout that is expected to be in the range of 2 million.

Still, it is always good to bank actual votes, especially in a race as volatile as this one. There is some anecdotal evidence, also, that polling firms that do not carefully account for early voting may understate its impact. Some polls suggested, for instance, that Barack Obama would defeat Hillary Clinton in the Democratic primary in California 2008, but Ms. Clinton prevailed there in part because she had built up a considerable advantage in early voting.

At least one more Florida poll is likely to be released before the end of the day. The polling firm Public Policy Polling, which has had strong results in the contests so far and which sometimes posts teasers about what it is seeing in the race, said in a Twitter post on Sunday that it had Mr. Romney roughly tied with Mr. Gingrich in its interviewing in Florida rather than giving Mr. Gingrich the lead there. However, these results have not yet undergone demographic balancing, and the firm has additional interviews to conduct before it releases its results tonight.

Mr. Romney’s campaign, meanwhile, will presumably be employing every tool at its disposal in an effort to halt and reverse Mr. Gingrich’s momentum. Early on Monday, it released a pointed commercial that attacked Mr. Gingrich’s ties to Freddie Mac, an issue that could potentially be salient in Florida given the importance of the housing crisis there.

Still, Mr. Gingrich is not without advantages in Florida. He has generally polled well among older voters, which will represent a large fraction of the Florida electorate.

Mr. Gingrich, who has a more moderate stance on immigration than Mr. Romney, could also potentially exploit vulnerabilities that Mr. Romney has among Latino voters. In the Florida Republican primary in 2008, according to exit polls of the state, Mr. Romney beat John McCain 34-33 among white voters. However, Mr. McCain’s large advantage with Latino voters — among whom he defeated Mr. Romney by a margin of 54 percent to 14 percent — helped to swing the victory to him there.

This post has been updated to include new data from Gallup’s national tracking poll.