While several Republicans brushed aside the significance of the endorsement, saying they believed it had been in the works for weeks, others said they were more concerned by Mr. Obama’s ability to dwarf Mr. McCain in spending during the final weeks of the campaign. As strategists for Mr. Obama eyed intensifying their efforts in Georgia, North Dakota and West Virginia, Republican advisers were trimming their efforts back to states won by Mr. Bush in 2004 and hoping for the best elsewhere.

The endorsement and the new fund-raising figures came on a weekend filled with signs of fresh momentum for Mr. Obama. He broke his record for crowd sizes, drawing an estimated 100,000 people to an outdoor rally in St. Louis. But the campaigns appear to be locked in a bitter duel in Missouri and a few other states that supported Republicans in recent presidential contests.

“The numbers we’re seeing at rallies are good portents,” said Mr. Obama’s chief strategist, David Axelrod, who like other aides reflected a sweaty and nervous optimism. “These become good barometers of enthusiasm.”

Using Mr. Obama as a magnet, the large rallies are specifically intended to stir interest in early voting and to serve as practice exercises for local campaign organizers to get out the vote on Election Day. By focusing on several Republican-leaning states, like Missouri, North Carolina and Virginia, advisers said they were striving to pave several distinct routes to an Electoral College victory instead of relying on a set number of states as Democrats have in the past.

While Mr. Powell said he had no plans to campaign for Mr. Obama, he became the highest-profile Republican to add his support to the Democratic ticket. He dismissed the notion that he was supporting Mr. Obama because they are both black. He contributed $2,300 to Mr. McCain last year and said he had studied both candidates for nearly two years, but in recent weeks Mr. Obama had impressed him.

“He displayed a steadiness, an intellectual curiosity, a depth of knowledge and an approach to looking at problems,” Mr. Powell said, adding: “Not jumping in and changing every day, but showing intellectual vigor. I think that he has a definitive way of doing business that would serve us well.”

Advisers to Mr. Obama said they had no immediate plans of using Mr. Powell in campaign advertisements, but that could change over the next two weeks. Mr. Obama, who has used his large campaign treasury to buy 30-minute spots on the television networks in the final week of the race, could highlight the endorsement at that point, but aides also said they expected Mr. Powell’s words to receive plenty of attention on their own.