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Supercommittee member Rep. James E. Clyburn told “Fox News Sunday” that, while he supports “about two-thirds of what Patrick J. Toomey has put on the table,” numbers in the plan might not add up.

“What I’m not for is trying to count something that CBO will not score,” the South Carolina Democrat said. “The legislation is very clear. We’ve got to come up with a plan that CBO will score, not that Pat Toomey will dream about.”

Mr. Clyburn said that while he was “very comfortable” about the prospect of a compromise, “I am not as certain as I was 10 days ago.”

Sen. Mark R. Warner, a member of another debt-reduction group known as the “Gang of Six,” urged the supercommittee to continue its work. But should it fail, the Virginia Democrat said, Congress should consider $4 trillion deficit-reduction proposals already floated by his group and by former Sen. Alan Simpson and Erskine Bowles, who was chief of staff for President Clinton.

“We’ve tried this congressional process. I think we need to let that play itself out,” Mr. Warner said on “State of the Union.” “But if they’re not successful, we think that the Simpson-Bowles approach or the Gang of Six, something that has got the $4 trillion number, ought to get a vote as well.”

Because the supercommittee talks have been conducted almost entirely behind closed doors, it’s difficult to gauge the process. But the overall tone of lawmakers on the panel in recent days hasn’t been optimistic.

Asked Thursday how he would characterize the talks, Sen. Rob Portman, an Ohio Republican and a panel member, paused and said, “Honestly, I’m not sure. I’m not sure.

“When you get closer to the end, it’s harder to be optimistic; but I’m remaining hopeful,” he told a gathering of reporters at the Capitol.