The list of leaders urging the 12-member debt supercommittee to aim higher than its $1.5 trillion mandate grew longer Monday with a letter to the panel’s members by five dozen prominent former lawmakers, officials and business executives.

Those signing the letter include four former co-chairs of federal deficit-reduction commissions — Alan Simpson, Erskine Bowles, Alice Rivlin and Pete Domenici — as well as economists, former administration officials and former lawmakers of both parties, such as former Treasury secretary Robert Rubin, former senator George Voinovich (R-Ohio), former New Jersey governor Christine Todd Whitman (R) and Moody’s Analytics chief economist Mark Zandi.

“We believe that a go big approach that goes well beyond the $1.5 trillion deficit reduction goal that the Committee has been charged with and includes major reforms of entitlement programs and the tax code is necessary to bring the debt down to a manageable and sustainable level, improve the long-term fiscal imbalance, reassure markets, and restore Americans’ faith in the political system,” the leaders write in the letter, which was spearheaded by the Committee for a Responsible Federal Budget.

The group announced the letter at a news conference at the National Press Club on Monday afternoon. The New York Times first reported on the letter Monday morning.

Leaders of both parties on Capitol Hill have been urging the panel to agree to a plan to achieve more than $1.5 trillion in deficit savings over the next decade. President Obama added his voice to the mix late last Thursday when he said during his address to a joint session of Congress that the committee ought to find additional savings to pay for his $447 billion jobs plan, the details of which the White House sent to Congress on Monday night.

But just as in the negotiations earlier this year on raising the country’s debt ceiling, the gap between the parties in the current spending debate is over not how much to trim from the national debt but rather how to go about finding those savings. Now, as then, Democrats and the White House favor an approach that includes tax increases as well as spending cuts, while Republicans insist that any long-term debt-reduction plan not include tax increases.

That means that, while the panel has a fast-track authority that eluded previous debt-reduction efforts, leaders still face a tough task ahead of their Thanksgiving deadline.