President Obama’s once-commanding fund-raising advantage is declining as major Republican donors rally for Mitt Romney, conservative “super PACs” far outpace their liberal counterparts and tax-exempt issue-advocacy groups swarm the political landscape.

With Republican Congressional committees largely keeping pace with the Democrats, Mr. Obama and his Democratic allies appear increasingly likely to be matched or even outspent by Republican and conservative groups — a far cry from 2008, when Mr. Obama outspent Senator John McCain by more than two to one and super PACs did not exist. All told, Mr. Obama, Congressional Democrats and liberal groups have raised at least $547 million in this election cycle while Mr. Romney, Congressional Republicans, and the top conservative outside groups have raised at least $462 million, according to a review of reports filed with the Federal Election Commission through Sunday and interviews with officials.

Those figures do not account for all the money spent through tax-exempt issue-advocacy organizations that are not required to disclose their spending, like Americans for Prosperity, founded by the billionaire Koch brothers, who have reportedly pledged to steer at least $200 million to conservative advocacy groups by Election Day. Nor does it include money spent directly by unions this cycle.

But even as the gap narrows, Republican and Democratic expenditures reflect starkly different bets on how to best spend money in the race’s first phases. Mr. Obama has used his early cash reserves to finance the creation of a massive political and technological infrastructure, placing field workers in swing states and using sophisticated data-mining techniques to rebuild his winning coalition from 2008. And while Mr. Romney battled for the Republican nomination, conservative super PACs and issue groups spent most of their millions on advertisements pounding Mr. Obama.