By: CNN's Dana Bash and Ted Barrett and Rachel Streitfeld

Washington (CNN) – Republican Sen. Tom Coburn, R-Oklahoma, is pulling out of so-called "Gang of Six" bipartisan negotiations for now – dealing a potentially fatal blow to the group that has been working intensely for some five months to find agreement on a deficit reduction plan.

Coburn believes the group has reached an "impasse."

"When you're at an impasse the worst thing you can do is keep trying to hit your head against the wall until you break your head," Coburn said. "I'm just going to give it a rest for a while and see. They all worked in good faith, every one of them and so it's not a matter of bad faith, it's a matter of being a realist about what you can accomplish and what you can't.”

Coburn said he is taking a break from the talks and "not attending right now," but insists he still considers himself a member of the Gang of Six.

But the reality is that the success of any deal the group would come up with depends largely on having support from someone like Coburn, with strong bona fides among conservatives.

Sen. Saxby Chambliss, R-Georgia, another conservative member of the bipartisan group, acknowledged Coburn's withdrawal will kill their efforts in the short-term.

When reporters asked Chambliss if there could be agreement without Coburn, he replied, "no."

"I would hope that eventually we'll still, as a group of six, be able to come together on some long-term resolution of the issue but looks like it's not going to happen short-term," conceded Chambliss, who said they will get together Wednesday to see where they are.

The goal of the group, which has been meeting regularly since January, is to cut $4 trillion from the deficit over 10 years.

Coburn said the main reason he believes the talks are stalled is because of differences over how to rein in costly entitlement programs.

"If we don't address those issues, you haven't fixed the problem," Coburn said.

Senators and aides involved in these negotiations are very secretive about the details of their discussions, but two Democratic sources familiar with the talks told CNN that Coburn proposed cutting an additional $130 billion from the Medicare program over the $400 billion the president's deficit commission proposed.

The Democratic sources say Democratic negotiators rejected Coburn's idea, and suggested that's what drove Coburn's decision to step away from the talks.

A Republican source familiar with the negotiations insisted to CNN Coburn did not make that specific proposal, but did say he was frustrated that Democrats were not giving more on cutting Medicare spending.

The source told CNN that Coburn had actually tentatively agreed to about $1 trillion in revenue raisers – meaning tax increases – anathema to most Republicans.

Coburn said he told Sen. Dick Durbin, D-Illinois, a Democratic member of the group, Tuesday morning, that he felt they had reached an impasse and would stop attending meetings.

"Look, we've been doing this a long time and we have a lot of things agreed to, a lot of pain for both parties but its got to be something that I think that will actually make some significant differences and we're just not there to what I think I can sell," Coburn said.

Earlier Tuesday, the third Republican member of the group – Sen. Mike Crapo, R-Idaho, told CNN he thought it was too early to tell if they would find consensus on a deficit reduction plan, calling it a "roller coaster."

But he was not nearly as pessimistic as Coburn.

"We're in some intense negotiations right now, but we're also at a point where things could come together," Crapo said.

Some of the wind was taken out of the group's sails after the deficit reduction talks shifted to a group led by Vice President Joe Biden, but senators continued to meet regularly.

Democratic senators in the group are Sen. Dick Durbin of Illinois, Sen. Kent Conrad of North Dakota and Sen. Mark Warner of Virginia.

Warner vowed to keep working.

"Our fiscal challenges are too great to stop working toward a comprehensive, bipartisan solution. I intend to keep working in good faith on these issues because we have made too much progress to stop now," Warner said.

NOTE: A Democratic bill to end oil subsidies is defeated in the Senate Tuesday.

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