Mr. Obama’s communications director, Robert Gibbs, said hours later that Mr. Obama had called her and told her “she would be a terrific candidate.” But aides were distraught days later when Mr. Obama told the CNN anchor Anderson Cooper that his experience outstripped hers, lowering him to a back-and-forth over experience with the No. 2 candidate on the Republican ticket. Mr. Obama would not go there again.

For more than a week, Ms. Palin’s star rose, along with Mr. McCain’s poll numbers, as Mr. Obama’s campaign seemed uncertain how to respond. Seeking to avoid the appearance of coming on too strong, the campaign stood by as she accused Mr. Obama of “palling around with terrorists.” Democrats on Capitol Hill were as nervous as they had been all year.

Mr. Margolis, the advertising strategist, got an earful when he was dispatched to give a briefing at the weekly caucus of Senate Democratic leaders.

“I said, ‘I’m just asking everybody to be patient for a couple of weeks,’ and there was a groan that went through part of the room, and it was, ‘Don’t sit back and allow this race to get away from you guys,’ ” Mr. Margolis said.

Confusing voters on experience issue
As had been their way so often, Mr. Obama’s aides did not change course because of the second-guessing. They were confident that with the selection of Ms. Palin, who had little national or international experience, Mr. McCain would ultimately confuse voters who for months had heard his argument that experience mattered most.

“I was trying to communicate pretty clearly that nobody was sitting back, but we believed that we had a strategy that was sound, that there was now real dissonance between the McCain message of August and the McCain message of September,” Mr. Margolis said.

Indeed, as Ms. Palin began to suffer a variety of wounds — poor interview performances with Charlie Gibson of ABC and Katie Couric of CBS, a brutal impersonation by Tina Fey on “Saturday Night Live” — they kept their distance and let it play out.

But they let no charge go unanswered in Mr. McCain’s television commercials, quickly producing responses to a barrage of attacks on a range of issues, including Mr. Obama’s ties to Mr. Ayers.

But by then, Mr. Obama’s financial advantage was drowning out most of what Mr. McCain was trying to say. Mr. Obama’s campaign ran four advertisements to every one from Mr. McCain.

But Mr. McCain’s spots were beginning to seem increasingly out of sync with the heightened public anxiety surrounding the financial crisis. The meltdown of the financial markets ultimately ended any hope of a comeback for Mr. McCain.

As Americans were increasingly worried about their futures, Mr. Obama’s message of help for the middle class and promise of steady leadership was resonating with the white, working-class voters he had been seeking to win over for nearly two years. He managed to cast his rival as out of touch and erratic, and repeatedly linked him with what he portrayed as the devastating policies of the Bush administration.

Crossing the racial divide
In the end, it appeared to work, helping him cross the racial divide and build support in swing states and some of those Democrats had long surrendered to the Republicans.

Aides were seeing gains in previously red states like Virginia, and comfortable leads in swing states like Pennsylvania. But they were mindful of the complacency they had shown in New Hampshire. In the pouring rain that had delayed the World Series, Mr. Obama decided against canceling a rally in the last days of his campaign, to show that he was still working for the win.