WASHINGTON >> Days after President Barack Obama called for forming a bipartisan group in Congress to begin negotiating a $4 trillion debt-reduction package, the parties have not even agreed to its membership. Yet six senators — three Democrats, three Republicans — say they are nearing consensus on just such a plan.

Whether the so-called Gang of Six can actually deliver something when Congress returns from a recess in May could determine whether Democrats and Republicans can come together to resolve the nation’s fiscal problems before the 2012 elections.

As Obama and Republican leaders have warred publicly over the budget, this small group of senators has spent four months in dozens of secretive meetings in offices at the Capitol and over dinner at the suburban Virginia home of Sen. Mark Warner, a Democrat.

The senators have weathered criticism from bloggers and even colleagues, including the leaders of their own parties, who oppose tampering with Social Security or taxes. The gang nearly collapsed several times, including two weeks ago.

The oldest members of the group — Sen. Richard J. Durbin, 66, a liberal from Illinois who counts the Senate’s only socialist as a friend and ally, and Sen. Saxby Chambliss, 67, a genial Georgia conservative whose nasty first campaign left lingering bad feelings among Democrats, and who is a confidant of Speaker John A. Boehner — illustrate that even with the mounting federal debt intensifying the partisan divide over spending and taxes, the severity of the fiscal threat is forging unlikely alliances.

If Durbin and Chambliss can cut a deal on Social Security and new tax revenues, their associates say, then just maybe all of Washington can come together.

For Republicans, that means accepting higher taxes and lower military spending. For Democrats, it would mean agreeing to curbs on the unsustainable growth of Medicare and Medicaid spending, as well as tweaks to Social Security, to avert a big shortfall in 2037 and as a trade-off for Republicans’ support on taxes.

Durbin and Chambliss reached those conclusions last year, each confronting the widening annual gaps between projected revenues and spending as the population ages and health care prices rise.

Durbin, the No. 2 leader in the Senate, was on Obama’s bipartisan fiscal commission, which recommended some solutions. Chambliss had joined informally with Warner to host private tutorials for Senate colleagues of each party with experts like Ben S. Bernanke, the Federal Reserve chairman.

The two senators’ paths merged last December.

Several months ago, with Durbin as its most surprising yes vote, 11 of the 18 members of the president’s fiscal commission backed a blueprint to pare $4 trillion from projected deficits in the first decade. It would cut domestic and military spending; curb Medicare and Medicaid; and overhaul the tax code, limiting or repealing tax breaks and using the new revenues to lower tax rates and reduce deficits. Separate from its debt-reduction plan, the panel proposed benefit and payroll tax changes to stabilize Social Security for 75 years.

Immediately, Chambliss and Warner enlisted four senators from the commission majority to negotiate writing the recommendations into legislation. Besides Durbin, the others were Sen. Kent Conrad, the North Dakota Democrat who leads the Budget Committee, and Sens. Tom Coburn of Oklahoma and Michael D. Crapo of Idaho, both Republicans.

“As I said to a Republican recently,” Durbin said in an interview, “it’s like we’re on a long flight here and we’ve come so far there’s no turning back — we’ve got to land the plane.”

The group’s efforts hold peril regardless of the outcome. If successful, its plan could be taken up as Congress debates this spring over raising the nation’s $14.2 trillion debt limit, but the group is hardly assured of support from Senate colleagues, let alone lawmakers in the House, where Republicans, including dozens of new Tea Party sympathizers, refuse to consider raising revenues. Failure would probably signal doom for the broader bipartisan effort Obama wants.

“The question my closest political friends are asking is this: Why is a progressive like Dick Durbin voting for this deficit commission report?” he wrote. The answer: “Borrowing 40 cents out of every dollar we spend for missiles or food stamps is unsustainable.”

So, Durbin added, “when we engage in the critical decisions about our nation’s future budgets, I want progressive voices at the table to argue that we must protect the most vulnerable in our society and demand fairness in budget cuts.”

That has been his mantra with disappointed allies in labor, women’s groups and the Senate. Durbin, in the interview, cited a private meeting requested by Sen. Bernie Sanders, independent of Vermont, a socialist and “a good friend.” Their exchange, Durbin said, captured the increasing difficulty in being a good progressive “at a time of limited resources.”

Sanders said he respected Durbin for his good intentions. “But I think the direction in which he is going in working with some of the most very conservative members of the Senate is not correct,” Sanders said.

Critics suggest that Durbin is seeking a new role to counter the prominence of Sen. Charles E. Schumer of New York, his former roommate and now his rival in the Senate leadership. But Andy Stern, a former labor leader who was on the fiscal commission and opposed its report, defended Durbin, saying, “It’s classic Washington that we can’t imagine that someone does something because they think it’s the right thing to do.”

That is Chambliss’ claim as well. “I hear my critics; I pay attention to my constituents,” he said in an interview. “But you’ve got to do the right thing and what’s best for the country.”

© 2011 The New York Times Company