The most outspoken Republican member of the Senate's "Gang of Six" abruptly dropped out of bipartisan debt-reduction talks Tuesday, declaring negotiations at "an impasse."

Sen. Tom Coburn (R-Okla.) told reporters that the group, which until Tuesday had three Republican and three Democratic members, was talking “about the same things over and over and not getting any movement.”

“It’s just a recognition that we can’t get there,” Coburn said, suggesting that the emerging debt-reduction plan doesn’t do enough to control spending on federal retirement programs.

Coburn spokesman John Hart later issued a statement saying Coburn “is disappointed the group has not been able to bridge the gap between what needs to happen and what senators will support” and has decided to take “a break from the talks.

“He still hopes the Senate will, on a bipartisan basis, pass a long-term deficit reduction package this year,” Hart said in an e-mail. “He looks forward to working with anyone who is interested in putting forward a plan that is specific, balanced and comprehensive.”

The remaining members of the group made plans to meet again Wednesday, saying they will keep trying to agree on a plan to reduce projected borrowing by $4 trillion over the next decade. But the departure of Coburn, the most prominent of the three conservatives, deals a severe blow to the effort. And it puts additional pressure on the two remaining Republicans, who have already come under fire for their willingness to discuss a plan that raises additional tax revenue.

Aides to Sens. Saxby Chambliss (R-Ga.) and Mike Crapo (R-Idaho) said Tuesday that they remain hopeful that the now Gang of Five would be successful in its effort.

“The group is facing tough issues, but it has faced tough issues before and continued to work on the issue,” said Crapo spokeswoman Susan Wheeler.

“You have highs and lows in negotiations,” Chambliss said. “And sometimes we feel better than other times.”

The group — which also includes Democrats Mark Warner (Va.), Kent Conrad (N.D.) and Richard J. Durbin (Ill.) — has been meeting for months in to draft legislation based on the recommendations of President Obama’s fiscal commission. That plan proposed to reduce borrowing to about $5.4 trillion over the next decade by sharply cutting spending on domestic programs and defense, raising the Social Security retirement age and overhauling the tax code to lower rates while eliminating dozens of cherished tax breaks. The Gang of Six has been working on a framework that would instruct a variety of Senate committees to craft legislation to meet its debt-reduction goals.

Earlier this month, the group appeared to be tantalizingly close to agreement. But then, sources said, Coburn started bringing up new issues at every meeting, or demanding to reconsider old ones. For example, Coburn was pressing for sharper cuts to Social Security than had been previously agreed to, said sources familiar with the talks, who spoke on condition of anonymity because of the privacy of the negotiations.

During a three-hour session late Monday, sources said, other members of the Gang of Six told Coburn that they had to move on. Then on Tuesday, Coburn told them that he was dropping out.

Vice President Biden has convened a separate group of bipartisan lawmakers to put together a budget-cutting deal to help persuade a reluctant Congress to raise the legal limit on government borrowing. Meanwhile, supporters say they believe the Gang of Five will continue to press on.

“As long as they’re still in the room, hammering away at this effort, this is the model for what’s going to work,” said Maya MacGuineas, president of the bipartisan Committee for a Responsible Federal Budget. “So we have to hope it does.”

Also on Tuesday, Senate Majority Leader Harry Reid (D-Nev.) said that he is confident that a final deal on raising the country’s debt limit this summer will include a repeal of tax subsidies for the top five oil companies, a top Democratic agenda item.

“There is no justification for giving these companies that are making so much money, and their own executives said these subsidies are unnecessary, there’s no justification for continuing that,” Reid told reporters after Senate Democrats’ weekly caucus luncheon.

Democrats have been pushing for repealing subsidies for the five biggest oil companies, arguing that the move would not only lower gas prices but would also make a dent in the national debt. Republicans and the oil industry have contended that repealing the subsidies would increase costs for consumers and would have only a slight effect on the country’s debt.

The Treasury Department has projected that Congress has until Aug. 2 to raise the $14.3 trillion federal debt ceiling before the country risks default.

Staff writer Felicia Sonmez contributed to this report.