Democrats and many economists consider particularly urgent the need to extend jobless benefits and the one-year payroll tax cut. With national unemployment stuck at 9 percent, and the ranks of the long-term unemployed at record levels, the government is providing up to 99 weeks of support to about 3.5 million people.

Meanwhile, the payroll tax cut, enacted last December, allows most American workers to keep an additional 2 percent of their earnings, a boon to tight household budgets as well as the economic recovery. Economists at J.P. Morgan Chase recently estimated that if Congress does not extend the two measures, economic growth next year could take a hit of as much as two percentage points — enough to revive fears of a recession.

Time is also running out for doctors who see Medicare patients. These physicians are scheduled to absorb a 30 percent cut in government reimbursements in January. A long list of tax breaks, including an inflation adjustment that protects more than 30 million families from paying the alternative minimum tax, also will be eliminated unless Congress acts.

Although many of the expiring provisions have received bipartisan support in the past, this year they face a welter of political obstacles, none more important than cost.