But the White House on Thursday gave Republicans assurances the president would not employ a potent weapon to get what he wants. Some Democrats, including former President Bill Clinton, have theorized that the Constitution gives the president the authority to raise the debt ceiling unilaterally, citing a clause in the 14th Amendment guaranteeing that the nation’s debts “shall not be questioned.”

Mr. Obama renounced such an assertion of authority on Thursday through his spokesman. “I can say that this administration does not believe that the 14th Amendment gives the president the power to ignore the debt ceiling,” Jay Carney, the White House press secretary, told reporters, reading from language that had been prepared for him.

Administration officials had long discounted the possibility that the president would claim such power, but Thursday’s statement seemed more definitive than any in the past.

The exclusion of Senators Reid and McConnell and Ms. Pelosi, the House Democratic leader, is a departure from last year’s search for a major deficit deal. Then, Obama-Boehner deficit talks coincided with side talks between Vice President Joseph R. Biden Jr. and Representative Eric Cantor of Virginia, the House majority leader, which were followed by broader talks by a special bipartisan Congressional committee. All failed.

This time, while Mr. Boehner has made himself the sole focal point, aides say he has made sure a broad leadership team is behind him. He meets every morning while the House is in session with the full slate of Republican leaders, as well as the committee chairmen who would most likely implement a deal: Representative Paul D. Ryan of Wisconsin, who heads the Budget Committee; Representative Dave Camp of Michigan, who leads the Ways and Means Committee; and Representative Fred Upton of Michigan, who heads the Energy and Commerce Committee.

White House officials have begun daily conference calls with the communications staffs of Mr. Reid and Ms. Pelosi. The White House communications director, Dan Pfeiffer, met with the Senate Democratic Caucus last week, and the director of the National Economic Council, Gene Sperling, spoke with the House Democrats late last month.

The arrangement has led to bipartisan grumbling. Senator Bernard Sanders of Vermont, an independent and perhaps the Senate’s most liberal member, said on Thursday that Senate Democrats needed to find a way to make themselves more relevant to the search for a resolution to the fiscal standoff.