Representative Ron Paul of Texas, who has enjoyed a steady flow of grass-roots donations to his Web site, raised about $14.4 million in unitemized checks under $200, while Mr. Gingrich, who has a robust direct-mail operation dating back to his days in Congress, has taken in $8.8 million in such donations.

While Mr. Romney’s overall small-dollar total exceeds that of Rick Santorum, his closest rival, Mr. Santorum had the best month for small donors of any of the Republican candidates in January, capitalizing on his belated victory in Iowa to raise $2.6 million in contributions under $200. Millions more followed Mr. Santorum’s triple victories in Missouri, Colorado and Minnesota, according to his campaign.

Mr. Romney, who won in New Hampshire and Florida, took in just $1.2 million in small checks in January, suggesting that his steady accumulation of votes and delegates has not been accompanied by the sort of enthusiasm that yields spurts of grass-roots money.

The lack of small donations is one financial challenge facing the campaign. Another is spending: Mr. Romney spent $19 million in January, burning through much of the cash advantage he had accumulated in prior months. His campaign did relatively little advertising in advance of Super Tuesday, leaving the “super PAC” supporting him, Restore Our Future, to spend millions on ads on his behalf.

Those signs of spending restraint have revived speculation that Mr. Romney may need to tap some of his personal fortune in this campaign, as he has in previous campaigns for office. Asked at a news conference on Tuesday whether he would open his own wallet for the race, Mr. Romney did not rule it out.

“I don’t have any plans with regards to my campaign finances at this stage other than to keep on raising the money necessary to go forward,” Mr. Romney said.

Aides to Mr. Romney, a former Massachusetts governor, said any gap in small-check fund-raising had been overblown. Low-dollar collections have improved steadily, albeit without dramatic spikes, as the campaign has heated up, they said, with the number of contributions under $200 doubling from January to February.