WASHINGTON - A top Republican senator in the bipartisan "Gang of Six" seeking agreement on a plan to cut the deficit by $4 trillion over the next decade dropped out of the group Tuesday, saying his colleagues weren't willing to cut enough from benefit programs such as Medicare.

Sen. Tom Coburn of Oklahoma said that he did not see how the group could reach agreement and that he would stop participating at least for now.

"It's got to be balanced," Coburn said. "And I didn't perceive where we were was balanced."

The closely watched group has been working for months on a sweeping plan to wrestle the deficit under control with a mix of new tax revenues and cuts across a wide swath of the federal budget in hopes of breaking partisan gridlock.

The group will continue to meet without Coburn, several members said. But it's plain that major obstacles remain.

One reason the Gang of Six was noteworthy was that its GOP members - Coburn, Saxby Chambliss of Georgia, and Mike Crapo of Idaho - were willing to agree to revenue increases of $1 trillion over the coming decade as the price for getting Democrats to accept cuts to Medicare, Medicaid, and Social Security.

The group is united behind the idea - shared by President Obama's debt commission, budget experts, and several lawmakers - that reducing the deficit requires both more tax revenues and cuts to Medicare and Medicaid, whose costs threaten to swamp the budget in coming years.

But Senate leaders in both parties were cool to the effort, and Republican leaders in the House said they would never consider tax increases.

A congressional aide familiar with the talks and speaking on condition of anonymity said Coburn angered Democrats by bringing to the table eleventh-hour demands on new and immediate Medicare cuts that went beyond what the House Republicans had proposed in their budget blueprint.

Democratic Sen. Richard J. Durbin of Illinois, a member of the group, said it would meet again Wednesday. The other Democrats participating are Budget Committee Chairman Kent Conrad of North Dakota and Mark Warner of Virginia.

Another top Senate Republican, meanwhile, reported that separate talks - between the White House and congressional negotiators seeking agreement on deficit-reduction measures as part of a deal to raise the nation's debt limit - had identified $150 billion in spending reductions.

"You first look at the things where there might be some overlap between our budgets and their budgets," said Sen. Jon Kyl of Arizona, the Senate GOP's chief negotiator.

A possible agreement on $150 billion in budget cuts is a start, but far from the $2 trillion in cuts over several years needed to meet House Speaker John A. Boehner's demand that spending reductions be greater than the amount of additional borrowing authority needed to keep the country from default.

The nation hit its $14.3 trillion debt limit this week, but Treasury Department officials have indicated that they can maneuver resources to avoid default until Aug. 2.