WASHINGTON – A presidential commission will convene Tuesday at the White House to address what leaders of both parties agree is one of the greatest threats to the country’s economic future: the rising national debt.

Official forecasts suggest that without sharp changes in federal spending or tax collections, the United States could enter into a downward spiral of indebtedness that by the end of this decade would erode the country’s ability to educate its children, care for the elderly or mount a robust national defense.

Republicans and Democrats alike say the fiscal challenges have been too long ignored. But with the two parties feuding over health-care reform, Wall Street regulation and a host of other issues — and the economy still uncertain after a deep recession — there is considerable doubt that they could join hands to fend off a still-distant potential crisis.

“It would take a miracle,” agreed Senate Majority Whip Richard Durbin, D-Ill. But “I believe in miracles.”

Durbin is the highest-ranking lawmaker among a dozen members of Congress on the commission, which also includes six presidential appointees. The panel has until Dec. 1 to devise a plan to stop a federal borrowing binge that exploded during the recent recession and will worsen as retiring baby boomers tap into federal retirement programs.

The gulf between the two parties is vast. No budget commission has managed to spur action since 1983.

And a host of interest groups is lining up to rally the public against any solution that involves higher taxes or cuts to favored programs — particularly Social Security, which members of both parties consider the ripest target for compromise.

Panel members from both parties say the recent experience of Greece, deeply in debt and begging other countries to help pay its bills, provides a vivid incentive to set aside ideological differences and work together.