Obama "will have some time to prepare, and he's been doing some studying, but it is certainly less than we anticipated," campaign spokeswoman Jen Psaki told reporters.
She explained that "events in the Middle East," Obama's "busy travel schedule" and "just the constraints of governing" got in the way of debate prep.
Obama is to begin official preparations Sunday in Henderson, Nev., near Las Vegas, three days before the first 90-minute debate Wednesday.
That debate, at the University of Denver's 7,200-seat Magness Arena, is expected to focus on domestic policy, with questions likely dealing with the economy, healthcare, "the role of government and governing," moderator Jim Lehrer, executive editor and former anchor of the "PBS NewsHour," said last week.
The final two debates are to take place Oct. 16 in Hempstead, N.Y., and Oct. 22 in Boca Raton, Fla.
White House spokesman Jay Carney wouldn't explain Obama's debate preparations but indicated the president would work on making clear, concise answers to questions.
"The president is familiar with his own loquaciousness," Carney said. He pointed out that in a debate "you have a limited amount of time to answer a question, to give feedback, to push back on your opponent's answers. So that's certainly something he and all of us are cognizant of."
Psaki said she saw Romney as having "a lead on how prepared he is."
The former Massachusetts governor has been prepping for the debates for several weeks at his home in New Hampshire, at his Boston headquarters and while he travels, aides have said.
He huddled at a Los Angeles hotel part of last weekend with senior adviser Beth Myers and Sen. Rob Portman, R-Ohio. Portman took the role of Obama in mock debates, the Los Angeles Times reported.
But Myers, like Psaki, sought Thursday to lower expectations of her boss's performance. In a widely leaked memo, she said Obama enjoyed a "significant advantage" going into the debates.
"Obama is a universally acclaimed public speaker and has substantial debate experience under his belt," Myers' memo addressed to "interested parties" said.
Romney has "the issues and the facts on his side," she said, but "Obama is a uniquely gifted speaker" who "is widely regarded as one of the most talented political communicators in modern history."
She said Wednesday's debate would be Obama's "eighth one-on-one presidential debate," whereas it will be Romney's first.
Her memo didn't mention Obama's last presidential debate was four years ago while Romney participated in 19 debates with other Republican hopefuls during the rough-and-tumble GOP primary from June 13, 2011, to Feb. 22, 2012.
Myers also said, "We fully expect [Obama to use the debate as] a 90-minute attack ad aimed at tearing down his opponent."
By contrast, Romney would "talk about the big choice in this election -- the choice between President Obama's government-centric vision and Mitt Romney's vision for an opportunity society with more jobs, higher take-home pay, a better-educated workforce, and millions of Americans lifted out of poverty into the middle class," she said in the memo.
Psaki wouldn't preview Obama's debate strategy Thursday but told reporters the Romney people "fully expect" Wednesday's debate "is going to be their turning point. And we know people want to write a comeback-kid story."
When asked what the worst thing was that could happen to Obama during the debate from a campaign perspective, Psaki said, "Well, he could fall off the stage."
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