The maneuvering here is critical, because Mrs. Clinton’s aides, along with many Democrats not associated with her campaign, believe that her momentum will be difficult to slow if she wins in Iowa; polls suggest that she is strong in New Hampshire. The Clinton campaign has been flying in operatives from across the country to bolster the Iowa effort.

Complicating the matter even more, the campaigns of Gov. Bill Richardson of New Mexico and Senators Joseph R. Biden Jr. of Delaware and Christopher J. Dodd of Connecticut also have the potential to sway the race. If candidates do not reach a 15 percent threshold in each of the 1,784 precinct caucuses on Jan. 3, their supporters make a second choice, a procedure that Mrs. Clinton’s aides fear could favor Mr. Edwards or Mr. Obama.

In the final seven weeks of the race, all campaigns are increasing their efforts here, placing new advertisements and investing more resources. To fight the new push by the Clinton campaign, rivals are also planning to spend nearly all their time in Iowa in December, hoping to raise doubts about her candidacy. While the Obama and Edwards campaigns have been gradually building for months toward this moment, the Clinton campaign has bolstered its activity here in recent weeks, hiring 100 new workers to concentrate on a person-to-person drive to explain the quirky process of the caucuses, with a goal of having 50,000 in-home visits by Christmas.

More than 60 percent of those who have identified themselves as Clinton supporters, senior strategists say, have never participated in the Iowa caucuses. It is a far higher share than the campaign had been anticipating, which suggests that many of the reliable rank-and-file Democrats have chosen another candidate. So the Clinton campaign is working to expand its universe of supporters to women who have never participated.

“No one is going to give Hillary Clinton this nomination,” said Terry McAuliffe, the national chairman of her campaign, who has traveled to Iowa nearly once a week for months. “She’s going to have to earn it.”