THE RECONVENING Congress has an enormous amount of serious work to do by the end of the year. It will also have to dispose of some showy side issues on which members of both parties may be tempted to grandstand. The less of this the better. The time is shorter than it seems.

The agenda is being driven partly by the government calendar. The current fiscal year ends Sept. 30, and a lot of existing authority -- to spend, to regulate in certain ways, even to tax -- is scheduled to expire with it. It took Congress the full seven months before it went home for vacation Aug. 1 to adopt a budget declaring its fiscal intentions for the year ahead. Now come the bills to carry out these intentions. Only two of the regular 13 appropriations bills for next fiscal year have got as far as House-Senate conference. Six others have passed just the House. All must be enacted by Oct. 1 or Congress -- as in every year since 1975 -- will have to adopt a continuing resolution to sustain unfunded agencies. White House aides have indicated the president may use the veto to brush up his credentials on the spending issue.

The two houses have also committed themselves to pass reconciliation bills, wrapping up the spending cuts envisioned in the budget resolution, in such politically difficult and technically complex areas as college student aid, veterans' health benefits, Amtrak and other transportation subsidies and Medicare.

In theory Congress must then also act by Oct. 1 on a new farm bill (which is intertwined with the budget resolution) and an extension of the expiring Superfund program to clean up buried toxic wastes. The House must act on the defense authorization bill, the Senate on the bill imposing sanctions on South Africa. Trade legislation is simmering in both houses. Also to be dealt with are the annual housing authorization bill, immigration reform, the year's major civil rights bill (reversing the Supreme Court's 1984 Grove City decision on the reach of civil rights laws) and a Dec. 31 deadline to fashion a new retirement program for the civil service. All serious business.

The side issues will likely entail further efforts by members to distance themselves from the deficit. There continues to be pressure, chiefly from the president and in the Senate, to vote on a balanced budget amendment to the Constitution. There may be the usual posturing in the Senate over raising the debt ceiling. Continuing resolutions almost always tempt some legislators to indulge in brinksmanship and threaten to shut down the government.

The White House also continues to press ahead with the president's tax reform proposal, which aides still describe as his first priority for the remainder of the year. Anything can happen in Congress. But so much time has elapsed and so many other issues remain that it is no longer clear that Congress can effectively take up this crushing subject up this year. Comprehensive tax reform would powerfully affect the entire economy; the bill cannot be flipped through quickly. In his earlier years in office the president pretty well kept control of Congress' agenda, and kept it mostly simple and clean. This year is more complicated.