Obama issued a fundraising appeal via email Monday in which he told supporters, "Listen, this race is tied" and said the outcome would determine the country's future for decades.
"That's what I'll be fighting for up on that stage tomorrow night — but I can't do it alone," he added.
Romney's campaign released its latest fundraising report, showing the Republican raised more than $170 million in September, slightly behind Obama's $181 million haul for the month. But Republicans have been energized by Romney's strong debate performance two weeks ago, and his top donors are holding a three-day retreat at New York's Waldorf Astoria hotel to talk strategy.
There's been no letup in the pace of activity in the nine battleground states whose electoral votes will decide the election: Colorado, Florida, Iowa, Nevada, New Hampshire, North Carolina, Ohio, Virginia and Wisconsin.
Mrs. Obama campaigns in North Carolina on Tuesday before heading to New York to watch the debate. She told NBC's "Today" show in an interview aired Tuesday that she's always "primed" as she sits in the audience, in case her husband looks at her for encouragement.
"I'm perched, I'm looking at him, I'm smiling, I'm giving a thumbs up if he can see it — with the lights you just never can tell," she said. "I assume that he can so I make sure that I'm always giving him that positive love."
Romney is hoping to keep his momentum going with another solid debate performance. Recent national polls show likely voters about evenly divided, but multiple surveys have detected increasing enthusiasm among Romney backers.
"The debate was huge and we've seen our numbers move all across the country," Ann Romney told Philadelphia radio station WPHT.
Now Obama is looking for the same kind of boost from a comeback performance.
"The president is his own harshest critic and he knows that Mitt Romney had a better debate," said campaign spokeswoman Jennifer Psaki. She described the president as "calm and energized and just looking forward to getting to New York" for the debate.
Body language expert Patti Wood said the first presidential debate and its vice presidential counterpart showed the importance of projecting a strong image to viewers.
To counter Romney's forceful debate performance, she said, Obama needs to work on projecting "alpha" non-verbal signals, conveying that "he really wants to move forward, he really wants to win, he really wants to continue to be president."
Biden, she said, overcompensated for Obama's lackluster performance and "went way over the top on volume level and aggressive interruption, rabbit-like jumping-in behavior."
Tuesday's debate audience of uncommitted voters was selected by the Gallup Organization. Moderator Candy Crowley of CNN will choose those who get to speak, after reviewing proposed questions to avoid repeats.
The final debate of the campaign will be Oct. 22 at Lynn University in Boca Raton, Fla., focusing on foreign policy.
Associated Press writer Nedra Pickler in Washington and Julie Pace in Williamsburg, Va. contributed to this report.
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