The warning came from David Plouffe, President Obama’s top political adviser: The Koch brothers and Republican “super PACs” have pledged hundreds of millions of dollars to defeat Mr. Obama, he told a dozen wealthy Democrats gathered in a Silicon Valley office suite. Do not believe what you read about all the money the president will raise himself, Mr. Plouffe urged them. He needs your help.

With the general election campaign just a few months away, Mr. Obama’s allies are under growing pressure to raise money rapidly for Democratic-leaning independent groups, warning his supporters that the huge cash advantage mustered by Republican groups could prove decisive this fall, overwhelming Mr. Obama despite his own formidable fund-raising apparatus.

Early indications are that the Democratic super PACs are facing an uphill climb. While Mr. Obama’s decision last month to endorse super PAC fund-raising — a reversal of his longstanding opposition to campaign spending by independent groups — has made potential donors more receptive to Priorities USA Action and similar groups, few so far have written the kind of six- and seven-figure checks that Republican super PACs are collecting.

Priorities USA Action, a super PAC supporting the president’s re-election, will report $2 million in February donations, group officials said, including $1 million from the television host Bill Maher. Those amounts are substantial for Priorities USA and a related group, which together raised about $6.1 million through the end of 2011.