Despite the "guillotine" of the Gramm-Rudman-Hollings budget-balancing law that awaits them next fall, Congress and the White House have gotten off to a slow, aimless, almost business-as-usual start in their pursuit of a fiscal agreement.

At both ends of Pennsylvania Avenue, officials suggest that events could conspire to reduce pressure for a budget compromise that would avert the sweeping October spending cuts threatened by the new budget-control law.

Unlike last year, when Senate Republicans seized the initiative on deficit reduction from the start, there is no concerted effort to begin the process of producing an accord.

By the end of last week, there was only one sign of movement: House and Senate Budget committees had agreed to work jointly on preliminaries that often stand in the way of broader policy agreements on the budget, including a set of common economic assumptions. Some members are hoping that this preliminary work, if it produces agreement instead of new friction, will bring a climate for further bipartisanship on the budget.

So far, no such bipartisanship has been evident. Legions of administration officials have gone from committee to committee to make their case for President Reagan's two-week-old fiscal 1987 budget, offering few if any concessions. House Democrats are also concentrating on Reagan's budget, hoping to extract maximum advantage from its political sore points. Senate Republicans seem content to let Budget Committee Chairman Pete V. Domenici (R-N.M.) lead the charge himself for tax increases as part of a deficit-reduction package.

"The economy's doing well, housing starts are up, unemployment is down, people aren't scared at all," said a Senate Budget Committee aide. A few weeks ago, "we all knew we were going to have to get down to serious business, but now I'm not sure," said a White House aide.

The pace of the budget process has a string of explanations:

A federal court knocked out the automatic trigger for the Gramm-Rudman-Hollings spending cuts, and many lawmakers think that the Supreme Court will uphold the decision, probably by early summer. While the law requires Congress to enact legislation putting the cuts into effect if they are no longer automatic, the ruling has eased the sense of urgency.

Probably more important are signs -- though tentative -- that deficits are on a downward slide, even without the cuts that would be mandated under the new budget law. Reasons include optimistic expectations about the economy over the next few years and an assumption that Congress will continue to clamp down severely on spending.

Domenici and Sen. Lawton Chiles (D-Fla.), ranking Democrat on the budget panel, warned that evidence the deficit is shrinking is largely illusory, created in part because of numbers-shuffling in budget calculations. "Like anybody else on a diet, the federal waistline doesn't get any smaller just because the scales are off," Chiles said.

The Congressional Budget Office estimate of annual deficits declining from more than $200 billion to just over $100 billion by fiscal 1991 were based on an assumption of steady economic growth, coupled with a no-growth policy for defense as well as domestic spending. Many lawmakers consider this unlikely, and Federal Reserve Board Chairman Paul A. Volcker cautioned that deficits would fall as projected only if spending is "very constrained."

But for the time being, the Office of Management and Budget and the CBO anticipate that deficit reductions of $37 billion to $38 billion will be necessary to meet the Gramm-Rudman-Hollings deficit target of $144 billion for fiscal 1987 -- considerably less than some lawmakers were anticipating when Congress convened a month ago.

One problem, Senate Republicans and House Democrats agree, is that the Reagan budget underestimates the dollar flow of defense spending by about $15 billion, meaning the deficit would be higher by that much if they are correct.

Another is that good economic news tends to whet Congress' appetite for spending, especially in an election year. Even as Congress faces an initial $11.9 billion bite under Gramm-Rudman-Hollings at the end of this month, pressure is building for 1986 supplemental appropriations, including programs from farm credit to U.S. embassy security, which will only add to the deficit problem next year.

Even if cutbacks are held to less than $40 billion, administration insistence on defense spending growth and congressional reluctance to cut domestic programs in an election year could make that target difficult to achieve without tax increases, many lawmakers say.

But Reagan, annoyed by charges that his budget was "dead on arrival" on Capitol Hill, has responded that a tax increase would be "vetoed on arrival" at the White House.