Since January, six senators have engaged in difficult negotiations and made painful concessions in a politically dangerous quest for something that has long eluded Washington: a bipartisan compromise to control the nation’s mounting debt.

By Tuesday evening, however, the “Gang of Six” was on the verge of collapse. Sen. Tom Coburn (R-Okla.) withdrew from the bipartisan working group, saying the senators simply could not overcome the polarizing political pressure that each faces. The group’s two other Republicans said it would be hard to continue without Coburn.

“The debt is still $14 trillion. It’s got to be solved in a bipartisan way,” Sen. Saxby Chambliss (R-Ga.) told reporters Tuesday night. “I hope that we’ll eventually, as a Gang of Six, be able to come together on some long-term resolution of the issue. But it looks like that’s not going to happen in the short term.”

Coburn’s withdrawal left Chambliss exposed, pushing him into the position as the group’s highest-profile conservative. He would be responsible for selling any compromise to those Republicans who think “compromise” is a dirty word.

If anyone could do it, it would be Chambliss. He arrived in Congress as one of then-House Speaker Newt Gingrich’s foot soldiers. Now in the Senate, he has one of the most conservative voting records.

A mild-mannered lawmaker in a Capitol crowded with lightning rods, Chambliss, 67, is one of Speaker John A. Boehner’s few confidants.

But compromise itself has proved elusive.

Asked whether the Gang of Six will continue to meet without Coburn, Sen. Mike Crapo (Idaho), the group’s third Republican, was uncertain. “I don’t know,” he told reporters. “I’ve got to think that through.”

‘Mr. Centrist’

In these immoderate times, some of the louder voices in politics consider reaching across the aisle sacrilegious. Back home in Georgia, Chambliss has faced as strong a backlash as any of the other five in the Gang of Six.

For weeks, calls have rolled into Georgia’s conservative talk radio shows from voters upset that Chambliss, the man they trust in Washington, is searching for common ground with Democrats — even worse, that he’s open to raising new tax revenue.

Erick Erickson, a radio host, has been trying to stoke a backlash. He recently wrote a column on his blog,, titled “There Is Time to Take Out Saxby Chambliss.”

“I get calls on my show on a daily basis that he’s stabbing us in the back — that he’s being Mr. Centrist instead of the conservative he says he is,” Erickson said in an interview.

Last week in Macon, Ga., after Chambliss addressed the Georgia Republican Party’s state convention, more than a few delegates sat in their chairs without so much as a single clap. Never mind that Chambliss didn’t even mention the Gang of Six in his speech.

But however cool the reception, there were no boos.

In fact, a month earlier when he and a Democratic member of the group, Sen. Mark R. Warner (Va.), visited the Rotary Club of Atlanta to discuss their bipartisan work, they received a standing ovation from about 500 business executives.

“He’s trying to be the bottle of peroxide on the sore,” said Tom Perdue, a longtime Chambliss adviser. “And in the process, the sore is going to heal.”

‘We can’t get there’

The abrupt unraveling of the Gang of Six dealt a blow to both Republicans and Democrats who had been counting on the group to deliver a politically viable plan.

Earlier this year, 64 senators — 32 from each party — signed a letter supporting the Gang of Six, whose members are some of the Senate’s heaviest hitters. In addition to Chambliss and Coburn, the group includes Richard J. Durbin (D-Ill.), one of President Obama’s closest, and most liberal, allies; Kent Conrad (D-N.D.), chairman of the Budget Committee; Warner, who as the governor of Virginia negotiated with a Republican legislature to balance the state budget by raising taxes; and Crapo, a budget hawk who is close to Minority Leader Mitch McConnell (R-Ky.).

Chambliss suggested Tuesday that the group, at the very least, owes its Senate allies a report on its work.

Those close to the talks said trouble has been brewing for weeks. Earlier this month, the group appeared to be tantalizingly close to an agreement. But then, Democratic sources said, Coburn started bringing up new issues at every meeting, or demanding that old ones be reconsidered.

For example, Coburn began pressing for sharper cuts to Social Security than had been previously agreed to, according to sources familiar with the talks who spoke on the condition of anonymity because of the private nature of the negotiations. And during a three-hour session late Monday, the sources said, Coburn demanded deep and immediate cuts to Medicare that went beyond anything previously proposed.

On Tuesday morning, Coburn called Durbin to say he was dropping out. He later told reporters that the group was at an “impasse” and complained that Democrats were unwilling to do enough to cut spending, particularly on federal retirement programs such as Social Security and Medicare.

The group kept “talking about the same things over and over and not getting any movement,” Coburn said. His departure is “just a recognition that we can’t get there.”

The remaining members of the group plan to meet again Wednesday. But it was unclear whether they will be able to proceed.

“I’m disappointed, but we’ll continue to do our work,” Durbin said as he prepared to leave the Capitol late Tuesday. “We put a lot into this. . . . I think there are some valuable ideas there that can help us resolve a major national problem. And I hope we can do it on a bipartisan basis.”

Even if the group resumes meeting, the momentum it had earlier in the debate clearly has been overtaken by parallel negotiations at the White House with Vice President Biden.

Surviving attacks

Criticism of Chambliss, Coburn and Crapo from some quarters in Washington has been withering. Grover Norquist, president of Americans for Tax Reform and one of the GOP’s fiercest tax opponents, has derided the Gang of Six for months.

“It looked to me like it was doomed from the beginning,” Norquist said. “It’s not treason to try. But it’s like building unicorns in the back yard. I don’t think it can happen.”

As an ironclad conservative, Chambliss has survived such attacks before. The American Conservative Union gave him a 100 percent rating for his votes in 2010; his lifetime rating is 93 percent.

Chambliss’s willingness to talk with Democrats, friends said, should not be mistaken for a willingness to compromise his principles.

“Saxby is a very rooted individual,” said Sen. Richard Burr (R-N.C.), a close friend. “He knows what we need to do, and I think he’s reached a point where he’s said, ‘Enough is enough, it’s time to lay something out on the table.’ ”

Until Tuesday, Chambliss had shown no signs of backing down, calling his work on the debt the most consequential of his career.

Said Perdue: “If you told Saxby today you had a crystal ball and you could guarantee him, ‘Saxby, you’re gonna lose the election in 2014 if you stay with this Gang of Six,’ he would stay with the Gang of Six. Being a senator is not what makes him a man.”

Staff writers Paul Kane and Felicia Sonmez contributed to this report.