Forget about issues, politics, administration arm-twisting, constituent mail, campaign contributions and all the other things that are supposed to fuel the engines of Congress. What drives Congress is deadlines, preferably the ones that fall at midnight on the eve of a congressional recess.

Apparently unable to act except under pressure of deadlines, Congress sets them for nearly everything -- for extension of taxes, for funding government agencies, for continuation of the government's power to borrow. More and more, it sets them when it can't decide what else to do.

No matter that it misses most of the deadlines, as it will today when the target date for completion of a congressional budget resolution for fiscal 1987 will pass without either house having acted. Congress normally enacts penalties only for others who miss deadlines (such as taxpayers who skip their April 15 rendezvous with the Internal Revenue Service); it does not fine, jail or otherwise inconvenience itself.

Moreover, some deadlines are more important than others, and few mean what they appear to mean.

When a new fiscal year dawns Oct. 1 without enactment of appropriations or stop-gap funding, affected government agencies are supposed to shut down; some do, some don't. When the debt ceiling is breached, the government theoretically faces default; something, however, always materializes at the last minute to pull it back from the brink.

Many programs lurch from deadline to deadline, saved by temporary extensions, as happened in the case of the 16-cents-a-pack federal tax on cigarettes, which lapsed and was then extended a half-dozen times over the past few months. Others are on a kind of perpetual life-support. In fact, so many small programs are hanging in limbo that the House Ways and Means Committee is considering an omnibus extension bill to cover all of them at once; at last count there were 12 waiting to be rescued.

Amorphous as they may be, deadlines do serve a number of useful purposes.

They can force otherwise distracted lawmakers to focus, albeit briefly, on the issue at hand. They often provide a touch of high-noon drama and urgency, as well as a pretext for the kind of doomsday-style oratory that politicians love.

When they mean something, as in the case of "continuing resolutions" to fund the government in the absence of appropriations bills, deadlines provide leverage for consideration of issues that would otherwise be ducked, ranging from abor- tion to arms control. This is either democracy-in-action or legislative blackmail, depending on one's point of view.

At the other extreme, the imposition of deadlines can artfully camouflage indecision or delay in resolving an issue. "Congress Sets MX Vote for March" sounds a lot better than "Congress Ducks MX Vote Until After Elections," even though they mean the same thing.

If the delay is long enough, sometimes the problem will go away, or something will happen to make it easier to resolve. For instance, lawmakers torn over aid to antigovernment contra rebels in Nicaragua can almost always rely on the Sandinista regime to misstep on the eve of an aid showdown, making it easier to defend a procontra vote.

Finally, the frenzy of activity that accompanies deadlines provides a modest sense of accomplishment, even if resolution of the deadline crisis produces nothing more than the setting of another deadline.

To understand legislating by deadline, one need look no further than the Gramm-Rudman-Hollings budget-control law, which sets the deadline that Congress will be missing today. It is a living monument to Congress and its love affair with deadlines.

When Congress faced a deadline last fall for raising the debt ceiling after months of inconclusive efforts to rein in budget deficits, Sens. Phil Gramm (R-Tex.), Warren B. Rudman (R-N.H.) and Ernest F. Hollings (D-S.C.) held the debt measure hostage for passage of legislation that sets targets and deadlines for bringing the federal budget into balance over five years.

Not only was the bill born of a deadline crisis, but it sets out enough deadlines to keep Congress on a perpetual budget treadmill for the rest of the decade. It sets five years' worth of deficit targets, along with deadlines for meeting them. Moreover, nearly every month within each of the five years brings a new deadline, sometimes more than one, for step-by-step progress toward meeting the annual deadlines.

For most of the year, the deadlines are typically congressional, meaning that they can be missed with impunity. Today's deadline is one of these. But there are others that exact a price. For instance, the House cannot take a Fourth of July recess until it has approved appropriations and deficit-reduction legislation for the following year. And the Senate, at least theoretically, cannot consider spending bills until it has passed a budget.

More importantly, each deadline that is missed makes it harder to meet the deadline that really counts, when, in late summer, it is determined whether Congress has met its deficit target for the year ($144 billion for fiscal 1987). If the deficit is more than $10 billion over the target, the Gramm-Rudman-Hollings spending cuts take effect Oct. 15.

Although the new budget law raises the stakes for this year's round of deadline gamesmanship, a bystander might be forgiven if he didn't notice the difference on Capitol Hill as Congress proves once again this week that it can't live without deadlines -- or with them either.