WASHINGTON — Political heavyweights from past budget debates descended on the congressional “supercommittee” to deliver a tough message as the panel struggles to agree on a $1.5 trillion deficit-reduction plan by its Thanksgiving deadline.

The four veterans offered their expertise – along with some criticism – as they implored the committee to worry less about the partisan political climate and more about the economic harm that could come to the financial markets and nation’s credit rating if a big deal cannot be reached.

“I’m worried you’re going to fail,” said Erskine Bowles, the former Clinton administration official and co-chair of President Obama’s fiscal commission.

“The effect it would have on how people look at this country would be disastrous,” he said.

“Devastating,” agreed Alice Rivlin, the founding director of the Congressional Budget Office, who warned of a long period of stagnant economic growth, “worse than the one we’re climbing out of,” if global markets dismiss Congress as dysfunctional.

The 12-member supercommittee, six Democrats and six Republicans, has been meeting mostly behind closed doors to develop a proposal to cut deficits over the next decade.