WATERLOO, Iowa — Senator Barack Obama is seeking to capitalize on a moment of opportunity in the weeks before the Iowa caucuses to challenge Senator Hillary Rodham Clinton’s long dominance of the Democratic field, and in doing so, he now faces intensified questions about his vulnerabilities in a general election.
These days, Mr. Obama spends less time acknowledging Mrs. Clinton as he speaks to Iowans. But he finds himself at the center of a fusillade of criticism from his rivals, including an assertion by former President Bill Clinton that electing Mr. Obama would be “rolling the dice” for America — a comment that validates the political threat Mr. Obama poses.
Mr. Obama, in an interview on Friday, addressed the shift in sentiment about his prospects of beating Mrs. Clinton in Iowa and holding her off in New Hampshire and other states that follow. “A month ago, I was an idiot,” he said. “This month, I’m a genius.”
The campaign of Mr. Obama, which slogged uncertainly through a period in the late summer and fall, alarming contributors who feared that he might have missed his moment, is now brimming with confidence as he delivers a closing argument to Iowa voters. His speeches are noticeably crisper, his poise is more consistent and many supporters say they no longer must rely upon a leap of faith to envision him winning the nomination.
With one week remaining before the campaign pauses for Christmas, Mr. Obama is dashing through a 22-city tour from the Mississippi River in the east to the Missouri River in the west, rushing to lock in voters before a holiday interlude. His organization faces its greatest test yet: turning enthusiasm among many grass-roots Democrats into widespread support at the caucuses on Jan. 3 in precincts that will decide the outcome, particularly rural areas where his support still remains uneven after 10 months of campaigning.
As he traveled across Iowa a month ago, a chief element of Mr. Obama’s pitch was to draw sharp contrasts with Mrs. Clinton and to urge voters to consider whether she had been truthful in explaining her positions. One of the few mentions he made about his rival here Saturday was to respond to criticism by associates of the Clinton campaign that he was too inexperienced and his background was unexamined.
“I understand that there’s a history of politics being all about slash and burn and taking folks down,” Mr. Obama, of Illinois, told reporters. “I recall the Clintons themselves calling it the politics of personal destruction, which they decried. My suspicion is that’s just not where the country’s at right now. They are not interested in politics as a blood sport.”
Yet despite a fresh sense of confidence surrounding Mr. Obama, the race in Iowa remains remarkably unsettled. Many potential caucusgoers are still making up their minds — or are open to changing them — as the six major Democratic candidates unleash a barrage of advertising that urge voters to consider the gravity of the election.
A variety of polls show Mr. Obama, at worst, to be in a dead heat with Mrs. Clinton in Iowa and strongly gaining on her in New Hampshire, which will have a primary election on Jan. 8.
Strategists for Mr. Obama said that they believed he had sufficiently answered questions about his experience. But fresh doubts are being injected into the atmosphere of the race every day. In an advertisement, another Democratic opponent, Senator Joseph R. Biden Jr. of Delaware, says that “being president is not the same as running for president.”
Still, Mr. Obama finds himself in the tightest competition with Mrs. Clinton, who dropped her above-the-fray posture and became more combative in recent weeks, but even more directly with John Edwards, a former senator from North Carolina who placed second here in 2004 and has staked his candidacy on a strong showing. Mr. Edwards released a new television advertisement on Saturday, in which he says, “Saving the middle class is going to be an epic battle, and that’s a fight I was born for.”
Reflecting concern about Mr. Edwards’s campaign, Mr. Obama briefly mentioned him at a campaign stop on Saturday in Independence when he responded to criticism from Mr. Edwards about Mr. Obama’s health care plan. Both candidates are fighting for many of the same voters, a point underscored by their travel itineraries practically mirroring each other.
With the war in Iraq having lost some of its intensity as a distinguishing point among Democratic candidates (several audiences barely applauded on Saturday when Mr. Obama stated his opposition to the war), other issues have come to the fore, like health care, the weakening economy and which candidate is best suited to beat the Republican nominee in the general election.
Mr. Obama is seeking to remind voters of his judgment, temperament and unifying approach to seize upon what many Democrats see as a moment of vulnerability for Mrs. Clinton. At the same time, he has narrowed his focus to a micro-level in Iowa, calling sheriffs, local officials and prospective precinct captains when he passes through town.
Before leaving Iowa for the weekend, Mrs. Clinton forcefully, if obliquely, pressed the case that she was not only more experienced than Mr. Obama, but better able to take on what is sure to be an aggressive campaign by the Republican nominee.
“I’ve been vetted,” Mrs. Clinton, of New York, told reporters on Friday. “I’ve been tested. There are no surprises.”
An adviser to Mrs. Clinton’s campaign suggested last week that Mr. Obama’s admission of drug use as a young man could weaken his candidacy. Her campaign repudiated the remarks, Mrs. Clinton apologized and the adviser resigned. But she and her aides have kept the issue alive by referring to it publicly in what appeared to be an effort to drive up negative views of his character and to raise doubts about his ability to weather a general election.
In an interview, Mr. Obama responded that voters would ultimately be turned off by such attacks on him, particularly about his admission more than a decade ago that he used marijuana and cocaine in his youth.
“My past and my character seemed to be fine when I was 20 points down,” Mr. Obama said. “Those kinds of tactics or strategies, I think, are emblematic of an old politics. It’s the exactly the kind of politics that the American people are tired of.”
In the final 18 days of the race here, Mr. Obama intends to devote nearly all of his time to Iowa, with the exception of a two-day trip to New Hampshire. He completed his final fund-raiser of the year on Tuesday in Seattle, which freed his schedule for 15-hour days of back-to-back rallies and town meetings, a pace far more hectic than much of the year.
“The political climate on the night of the caucuses is as important to turnout as anything,” said Mr. Obama’s campaign manager, David Plouffe. “Right now, we have a good climate, but the next 20 days will seem like 20 years.”
So in the closing weeks of the race, as volunteers make about 10,000 phone calls every night on behalf of Mr. Obama except on Christmas Eve and Christmas Day, they are given the task of collecting an answer to a new piece of information: If you are going away for the holidays, will you be back by Jan. 3?
In such a tight race, Mr. Plouffe said, a margin of 2,000 or 3,000 could tip the balance. That is why none of the candidates are leaving any possibilities to chance, from the students who will be 18 by Election Day in 2008 to the older voters who are being gently asked to stick around Iowa until the caucuses before moving to a warmer climate for the winter.
As he reaches out to those voters, Mr. Obama imposes upon them a heavy sense of responsibility. At the same time, he seems to unwittingly raise expectations for his own campaign here.
“You in Iowa have this extraordinary privilege of choosing who the next president of the United States is going to be,” Mr. Obama told an audience in Guttenberg on the opening leg of his bus tour. “Whoever wins this caucus is likely to win the nomination and is likely to win the presidency.”