This Tuesday may go down in history as the day Congress discovered, much to its surprise, that the budget process works.

That was the day after the Senate budget committee reported that, because the costs of some programs had been underestimated, federal outlays for the fiscal year nearly half gone will be $10 million above the ceiling fixed by the second budget resolution Congress adopted last fall.

That notice means any new spending bill for this year can be blocked on the floor by a single objection until Congress either cuts spending or raises the ceiling.

Two spending bills were knocked off the program immediately; one had been scheduled for action in each body Tuesday. A sizeable list of other supplemental spending legislation faces delays until Congress figures a way out.

It is the first time in the five years it has been in operation that the budget process has shown it can bite. Always before Congress set the spending ceiling high enough or delayed reestimates long enough to get the needed supplemental spending through before the trap snapped. Now, much to the surprise of critics who considered the budget process a glorified adding machine, the costcutting rules are coming into play.

What happened was that both adminstration and Congress made overly optimistic economic and legislative assumptions. A year ago the adminstration assumed inflation would run at 7.5 percent in 1979. It was 13 percent. It assumed 6.4 percent for the fiscal year ending next September 30. January's rate was 18 percent. The budget assumed passage of money-saving legislation such as hospital cost containment that didn't happen.

House Democrats had to squeeze the projected deficit below realistic expectations partly to get the budget resolution passed over Republican reservations. And the House last fall rejected Senate Budget chairman Edmund Muskie's (D-Maine) plea to go through the process of "reconciliation" as provided in the budget act to trim spending when it appeared certain to break through the budget resolution ceiling.

What to do? Congressional staffers think it unrealistic to expect Congress to make $10 billion or more in spending recissions with the year nearly half gone. What is expected to happen in about a month after a great deal of fussing over dollars is that Congress will pass a third budget resolution making room for essential spending. To get it passed, Democrats are expected to tie it to the first budget resolution for next year setting as a target a balanced budget.

Meanwhile, there is the threat of delay for money for draft registration, aid to Nicaragua, $136 million in disaster loans, $458 million for black lung disease benefits and $2.5 billion for food stamps.

Farther down the road, if the problem isn't resolved by June 1, the space shuttle program would run out of money and be unable to fly this year. There would be no funds for solar tax credits, Food for Peace or the Council of Wage and Price Stability that was supposed to prevent this kind of trouble.