"It's kind of like horse racing. If you're a horse that does well in the last sprint, you wait. But if you wait too long, there's going to be too much distance for you to catch up and you're not going to make it," said Steffen Schmidt, professor of political science at Iowa State University.

While supporters are impressed with Obama's signs of strength - his fund-raising prowess, the huge crowds at rallies, the Internet following - some are getting anxious that he hasn't risen in the polls.

"I am worried that he hasn't done more to close the gap," said a New York lawyer who has donated the maximum $2,300 to Obama's campaign and spoke on condition of anonymity. "It's positive that he hasn't fallen farther behind."

Preeta Bansal, a New York supporter, said she's not concerned.

"It's going to be a long fall. The race is just beginning and he's on fire," said Bansal, referring to recent stump speeches where she said he's been "superb."

New York City Councilman James Sanders, a Queens Democrat who supports Obama, conceded that supporters are "always concerned" and looking for the candidate to shine. But he said there's tremendous excitement for Obama, and real optimism that he can knock out Clinton.

"On the ground, the people, the groundswell is growing," Sanders said.

To overtake Clinton, the Obama campaign plans to drive home the message that the battle comes down to the following question: "Who do people think is most likely to bring about the change we need?"

David Axelrod, a top Obama adviser, said the Illinois senator won't hesitate to draw sharp contrasts with Clinton where contrasts are "germane and real." But don't expect a smackdown any time soon.

"I know there's a rooting interest, a kind of blood lust in the political community to see a kind of steel-cage match between Obama and Hillary," Axelrod said. "I don't think that's either politically smart or consistent with who he is."

The campaign has pinned much of its strategy on winning Iowa. Polls show Obama is in a close three-way race there with Clinton and John Edwards.

The campaign also has begun to deploy staff and build up ground operations in the states holding primaries on Feb. 5. And in the coming months, star supporter Oprah Winfrey "will do some things for us," Axelrod said.