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, and on Saturday evening The Des Moines Register’s editorial pages announced its endorsement of Mrs. Clinton. Many potential caucusgoers are still making up their minds  or are open to changing them  as the six major Democratic candidates unleash 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.