With John Edwards no longer in the presidential race, Senator Clinton will attempt to debate her way to the presidential nomination tonight in Hollywood’s Kodak Theater.

The departure from the presidential race of Mr. Edwards, who made his announcement public yesterday in New Orleans but did not make an endorsement in the race, sets the stage for a two-candidate duel between Mrs. Clinton and Senator Obama in the debate and in the more than 20 primaries on February 5. While the debate isn’t expected to garner the 70 million viewers that the seminal Nixon-Kennedy debate of 1960 did, it could draw a historic number of primary viewers, given its proximity to such a large number of important and delegate-rich contests as well as the drama of a showdown between two high-profile candidates. Mr. Edward’s absence will allow the candidates to direct their full fire on each other and will showcase the strengths and weaknesses of each.

It was unclear whom Mr. Edward’s decision would benefit. Mr. Obama’s supporters argued that those who backed Mr. Edward’s candidacy, heavy on a message of change, would be drawn to the relative newcomer to American politics. Mrs. Clinton’s advocates contended she would receive her fair share of voters due to her similarities on policy questions, such as health care, to Mr. Edwards.

One powerful ally of Mr. Edwards, a coalition of the 12 state councils of the Service Employees International Union, indicated it would not make an endorsement in the race any time soon. Even so, the Clinton camp hoped that the opportunity for a one-on-one face-off would help its candidate. “The debate promises to be a potentially defining moment in this primary battle,” a former chairman of the Democratic National Committee and supporter of Mrs. Clinton, Steve Grossman, said. “She will have a golden opportunity to break through the largely unflattering press coverage of the last few days.”

A Democratic political consultant who is neutral in the presidential race, Henry Sheinkopf, said the new dynamic could put more pressure on Mr. Obama. “There’s going to be a lot more focus on Barack Obama,” Mr. Sheinkopf said. “There’s no more filter on him. There’s no one to hide behind.”

Still, the private and public sentiment among Mr. Obama’s supporters was that Mr. Obama’s message of hope and change would resonate among those who were behind Mr. Edwards. “I hope it’s going to help,” Senator McCaskill, a backer of Mr. Obama, told CNN, describing Mr. Edwards’ voters as being “really hungry for change.”

A journalism professor at Northeastern University and author of “Presidential Debates: 40 Years of High Risk TV,” Alan Schroeder, said the one-on-one format could favor Mr. Obama. “Obama has not been as effective with a lot of other people on the stage,” Mr. Schroeder, who is neutral in the race, said. “He tended to get lost in the shuffle, especially in the early debates.”

Mr. Obama, campaigning in Denver, praised Mr. Edwards, and Mrs. Clinton’s campaign circulated a statement saying he had “lifted this campaign with his deep concern for the daily lives of the American people.”

While Mr. Edward’s populist message did not catch fire with American voters, he did win the backing of 12 state councils of the SEIU, including California, Idaho, Minnesota, and Massachusetts, which will hold their elections on Tuesday. The national coordinator of these SEIU state councils, Courtni Pugh, said the councils did not plan to make another endorsement. “We don’t have any plans to go anywhere,” she said. While the SEIU in California, which has more than 400,000 Democratic voters, has already distributed five mailings on behalf of Mr. Edwards, its Get Out the Vote phone banks will go unused in the days leading up to February 5.

Mr. Edwards was also the only of the so-called big three Democratic contenders who accepted public funds in his election. While he failed to raise the money of either Mr. Obama or Mrs. Clinton, he boasted a financial network of fund-raising trial lawyers, which is now up for grabs between the two remaining candidates.

“He has a very powerful fundraising base,” a chairman of Mr. Obama’s New England Steering Committee, Alan Solomont, said. “Any campaign would like to have their support.”

Mr. Edwards, who will retain the 26 delegates he won in Iowa, New Hampshire, and South Carolina, likely will be back in the public eye at the Democratic National Convention in Denver.

Keeping delegates permits a candidate to have their name called during a convention, negotiate a high-profile speaking slot at the convention, and even become a candidate again should disaster strike a political rival.