In the interview, Mr. Obama, of Illinois, acknowledged that he had held back until now, though he asserted it was a calculated decision to introduce himself in early voting states before engaging opponents. At times, he said, he has taken lines out of speeches prepared by his campaign that he felt were “stretching the truth.”

But Mr. Obama said the plan had always been for him to begin taking on Mrs. Clinton more directly in the fall. And he glared and said no when asked if he lacked the stomach for confrontational politics. “It is absolutely true that we have to make these distinctions clearer,” he said. “And I will not shy away from doing that.”

A test of just how far Mr. Obama is willing to go should come Tuesday night, when Democrats meet for a nationally televised debate in Philadelphia. [In a campaign statement issued Saturday in Iowa, Mr. Obama asserted that Mrs. Clinton had “repeatedly dodged opportunities to reveal her thinking about the best way to strengthen Social Security.”]

The interview came amid growing signs that Mr. Obama was looking for a fresh start for his campaign after nine months in which his aides said they were startled by the effectiveness of Mrs. Clinton’s campaign, and worried that her support was not as brittle as they had once believed.

Mr. Obama has built up his campaign war room, occasionally traveling with a speechwriter — reflecting concern of his aides that his public speeches tend to be long-winded — and begun spending more money on television advertising in Iowa and New Hampshire.

His senior aides said they were now spending much of their days fielding calls from concerned donors and other supporters asking why Mr. Obama was not challenging Mrs. Clinton more forcefully and warning that he could cede the role of the main anti-Clinton candidate to former Senator John Edwards of North Carolina, who is running an aggressive campaign in Iowa. Typically, one aide said, the supporter asks some version of the same question: “What happened to the Obama we saw at the 2004 Democratic convention?”

At the same time, aides said there was disagreement in the campaign about whether he should now begin investing all his time in Iowa, where polls show him to be running neck-and-neck with Mrs. Clinton and Mr. Edwards, hoping that a victory there would give him a lift in New Hampshire, where polls show him trailing.