A LAME DUCK session of Congress is, in theory, a bad idea. The session beginning Wednesday is exceedingly likely to demonstrate that it's also a bad idea in practice. The subject will mainly be money. It will be a frantic scramble to finish up the appropriations and budget bills -- the basic, routine responsibilities of Congress, postponed because the Senate Democrats didn't want to take the last hard votes before the election.

The legal deadline for the second budget resolution was Sept. 15, but, of course, it still hasn't been passed. Since that resolution sets the limits on spending and the deficit, it will require at least a tentative decision on the coming tax cut. The amount of highly detailed technical work still to be finished is enormous, and the strain will be augmented in the Senate by the change in party control. Pre-election procrastination, as it turned out, was not a highly successful campaign tactic.

At first glance, it seems odd that the triumphant Republicans, who campaigned on their own very explicit tax program, should now embrace the tax cut drafted last summer by the Democrats in the Senate Finance Committee. The Republicans say they want a tax cut as soon as possible, and they are ready to work with the Finance Committee's bill. But at second glance, perhaps it's not so odd after all. The Republican tax bill, with its broad and highly inflationary drop in income tax rates, scares a lot of solid conservatives. The bill's oversimplified rules for business depreciation do little for the businesses that need and deserve help the most -- while providing a bonanza for real estate speculation. A quick compromise with the Senate Finance Committee's bill offers the new administration an escape from the more embarrassing aspects of the famous Reagan-Kemp-Roth plan.

Legislation probably won't be actually enacted in this lame duck session. But this year's compromise bill would become the starting point for next year's action. The Finance Committee's bill would cut taxes more deeply than either Mr. Reagan or President Carter proposed. A large budget deficit for 1981 is now inevitable. As a purely partisan matter, there is an obvious advantage to the Republicans in getting it locked into the budget process while the Democrats are still at least nominally in power and responsible for it. What's in it for the Democrats? It's an opportunity to influence the next tax cut -- possibly to be the most important economic legislation of the Reagan administration -- while they still hold majorities in both houses.

The approaching duck appears to be not only lame but also disheveled and bad-tempered. In this unhappy session, the people urgently trying to pass a few last bills will be struggling to hold the people who are fed up and want to go home. Some Democrats may be tempted to procrastinate further and dump this year's budget quarrels, unresolved, onto the Reagan administration as it tries to get a grip on the 1982 budget -- the one that's published next January. But Congress can't do that without sacrificing an important part of its own control over the crucial budget process. Because the Senate Democrats wanted to dodge past the election before answering the hard questions, they must finish the budget now while their power drains away.