A Congress full of well-plucked lame ducks will flock back to town today for one last spin around the pond -- and possibly not much more.

With the agenda of unfinished business largely rewritten by last week's election results, the only certainty is that money measures -- the long overdue budget, spending plans and possibly a tax cut -- will dominate the 96th Congress until it adjourns.

So unexpected were the Republican takeover of the Senate and big gains in the House that it could take a day or two, at least, for the still shellshocked Congress to find its bearings. Until it does so, even the agenda will be in doubt.

At stake are bills covering Alaska lands, fair housing, nuclear and chemical wastes, revenue sharing, extra unemployment compensation, criminal code revision, reclamation policy, youth jobs, foreign aid, regulatory reform and many others that were pushed primarily by the Democrats -- along with a long list of judicial and administrative nominations that the outgoing Carter administration submitted to Congress before its defeat at the polls.

For some of these measures, it may be now or never.

The ambitious housing bill, which would strengthen the government's hand in fighting residential discrimination, cannot be expected to fare as well under the Senate Judiciary chairmanship of Strom Thurmond (R-S.C.) as it did under the chairmanship of Edward M. Kennedy (D-Mass.). And Reagan can be expected to view Alaska lands protection far differently than President Carter has.

For some dangling measures, there is strong pressure for immediate action. Reauthorization of revenue sharing for local governments, which expired Sept. 30, is a example.

However, House Majority Leader Jim Wright (D-Tex.) said yesterday that most of them -- along with the tax cut bill -- will probably get left behind as Congress pushes to adjourn before Thanksgiving.

In addition to tax and spending bills, Senate Majority Leader Robert C. Byrd (D-W.Va.) has listed the fair housing measure and criminal code revision as priority items for the lame-duck session, although their prospects are not considered good if they get caught in an adjournment rush.

The theory of lame-duck sessions is that the losers try to get all they can before time runs out. But it may not be all that simple this year."Why risk your neck in the last four minutes of a losing game when the season's been a disaster anyway?" groused a House Democratic staffer when asked about prospects for tax cut legislation.

Moreover, the House Budget Committee's endorsement of President-elect Ronald Reagan's promised 2 percennt spending cut and Wright's preference for deferral of tax legislation until next year indicate the Democrats will give fthe new administration some time -- and maybe a little rope -- to show how it intends to deliver on Republican campaign promises.

The last hurrah of the 96th Congress could be a long or short, depending in large part on whether Reagan and Senate leaders of both parties -- over the misgivings of the outgoing president and House leaders -- push ahead with the Senate Finance Committee's tax cut bill before the end of the year.

Reagan, who campaigned for a broader 10 percent cut in income taxes this year and in each of the next two years, has nevertheless given the go-ahead to a Senate Finance Committee-backed tax cut of $39 billion, including $22 billion for individuals and $17 billion for businesses.

Russell Long (D-La.) and Bob Dole (R-Kan.), respectively, of the Senate Finance Committee, have said they want to move ahead immediately with the bill. So have Senate Majority Leader Byrd and Minority Leader Howard H. Baker Jr. (R-Tenn.), whose roles will be reversed in January.

But the House Democratic leadership is against the idea, and House Ways and Means Chairman Al Ullman (D-Ore.), while saying he won't stand in the way of the Senate bill, has expressed doubt that final action on a tax cut is possible this year. So have some House Republican leaders. Moreover, a Carter veto is possible.

Another complication is the second budget resolution for fiscal 1981, which Congress is legally bound -- in theory at least -- to adopt before it adjourns. The budget resolution should have been approved by its target date of Sept. 15, but preelection skittishness over its anticipated hefty deficit, coming only a few months after Congress promised the country a balanced budget for the first time in 12 years, caused the resolution to be shelved.

Budget Committee leaders in both houses, fearful of the damage that further delay might do to the already heavily strained congressional budget process, don't want to go home again without a budget in hand. But if the job proves too big, Congress can waive the rules and go home empty-handed, devastating as this might be to the budget system.

There are also all those nagging appropriations bills that Congress left behind when it hit the campaign trail in early October and, failing to see the upheaval in its future, put most of the government on stopgap financing until it returned. Ten of 13 remain to be enacted, even though the government is two months into the 1981 fiscal year.

Some Republicans, who castigated the Democrats earlier for dillydallying on the appropriations bills, now favor more stopgap financing so Reagan and the incoming Republican Senate can leave their mark on spending for 1981. Such a delay could leave most of the government without congressional appropriations until near the end of the first half of the fiscal year, which would be unprecedented. But then the delay thus far has also been unprecedented, at least in recent years.

Hanging fire even longer have been about $10 billion in savings that Congress ordered as part of its first budget resolution last summer. The "reconcilation" package is in a House-Senate conference, which risks devastation to the budget process if it fails to complete its work.

Without these reconcilation savings and $10-to-$12 billion more in the second resolution, the 1981 budget deficit could approach the $50-to-$60 billion level of the projected 1980 budget, according to Sen. Pete Domenici (R-N.M.), the incoming chairman of the Senate Budget Committee.