When President Obama and Congress return to Washington this week, the countdown to the fiscal cliff will be measured in days -- yet no one knows how, when or even whether an agreement might reached.

Sorting out the scenarios is like trying to assemble a 100-piece puzzle on deadline. About the only decent bet is the one offered by Sen. Joe Lieberman, the Connecticut independent:

"We're going to spend New Year's Eve here, I believe."

There are four general ways the drama is likely to unfold: no deal at all, the long-elusive big deal, consideration of legislation that one chamber already passed or a small time-buying accord that would require further negotiations.

If no alternative is adopted, Bush-era income tax cuts will expire at the end of the year. On Jan. 2, $109 billion in automatic spending cuts would take effect, and half of them would involve defense. Those are just the big items, however.

Extended jobless benefits for an estimated 2.1 million Americans begin expiring at the end of the month. Also ending would be the 2 percentage-point Social Security payroll-tax cut and the alternative minimum tax "patch." The AMT, originally aimed at wealthy taxpayers who avoided high taxes with deductions, wasn't indexed to inflation, and therefore it could affect 31 million people unless Congress approves a change.

Also looming is the annual battle over the "doc fix," or Medicare payments to doctors. If no action is taken, those payments might be cut by about 27 percent.