Through the end of December, Mr. Romney had raised $2,500 from more than 14,000 donors, representing about 44 percent of all of his contributors. About 5 percent of Mr. Paul’s donors, 14 percent of Mr. Gingrich’s donors and 30 percent of Mr. Santorum’s donors had maxed out, meaning that a vast majority of their contributors can continue to give more money this year.

As Mr. Romney’s campaign prepares for a potentially long fight for the nomination, he has increased his fund-raising events in strongholds like Washington and New York and has begun prospecting elsewhere. During a campaign visit to Cincinnati on Monday, Mr. Romney made time for an evening stop at the Great American Tower, where local donors said they would raise more than $600,000 for him.

“We have exceeded our fund-raising goals and are on track with spending plans,” Spencer Zwick, Mr. Romney’s national finance chairman, said in a statement. “We are the only campaign who has the organization and resources to go the distance of a long primary process.”

Though other candidates appear to be capturing more grass-roots enthusiasm than Mr. Romney, they also ended January with far less money in the bank. Mr. Gingrich had about $1.8 million, and nearly as much debt. Mr. Santorum ended January with less than $1.5 million on hand, though his campaign said he raised at least $3 million in grass-roots contributions after his triple victories this month. Mr. Paul had about $1.6 million.

A super PAC founded by two former aides to President Obama, Priorities USA Action, raised just $58,816 during January, helping to prompt a drastic shift from Mr. Obama, who has long inveighed against the influence of outside groups. On Feb. 6, just days after the end of the January filing period, aides to Mr. Obama told his top donors that White House and campaign officials would appear at events for Priorities USA Action in a bid to increase its fund-raising.