Congress' fickle budget control process, which President Reagan so successfully exploited over the last two years, has turned around and bitten him.

Suddenly, instead of easing the way for Reagan's program, the process by which Congress each year reshapes the president's budget into a framework for its own future tax and spending decisions is threatening to dismember the program.

The wails once heard from the Democrats now come from the Reagan administration, especially Defense Secretary Caspar W. Weinberger, who seemed to like the process a lot better when he was winning.

The turnabout stems from the fact that, hidden behind all of the numbers, the budget is a reflection of political imperatives in Congress and perhaps the country as a whole.

Stripped of its arcane procedures that baffle even senior lawmakers, the budget process is how Congress forces itself to focus on priorities and weigh competing claims in terms of political pressures it is feeling from everywhere, especially back home.

The process has teeth that help make it more than just a theoretical exercise, including the painful "reconciliation" process, scheduled for use again this year, under which committees are required to make spending and tax adjustments to meet budget targets.

But its real significance may lie in the fact that it is the closest Congress ever comes to distilling its collective will and pouring it into one vessel, with a shelf life of a year.

With deviations along the way to reflect economic and political shifts, the budget decisions made by Congress early in a year--those being made now in setting tax and spending targets for the 1984 fiscal year starting Oct. 1--have a powerful impact on almost everything else it does, in concept if not in detail.

The hard shots taken by Reagan on the budget this year arise less from twists in the process than from a major shift in economic and political winds, whipped up by the prolonged recession and the perceived failure of Reagan's economic initiatives to tame it in time to avoid serious national suffering.

It also has to do with the clumsiness of the White House response to these changes, which were reflected in mounting Republican and Democratic boldness in challenging key elements of Reagan's program: deep tax cuts, a costly military buildup and heavy domestic spending cuts.

By failing to recognize the extent of congressional frustrations and pursuing a no-compromise strategy, the White House, according to many Republicans, has only itself to blame for budget blows it has received, culminating last Thursday in a virtually Democratic budget from the Republican-controlled Senate Budget Committee.

To the extent that the economy is rebounding and the White House appears finally to have been bludgeoned into moderating its no-compromise stance, Reagan may still be able to get a budget for fiscal 1984 that he can live with, albeit uncomfortably.

But what he faces right now are:

* A budget from the Democratic-controlled House that cuts his military buildup by more than half, restores billions of dollars cut from social welfare programs and jeopardizes the remainder of his tax-cut program, including the 10 percent income tax cut scheduled for July.

* A budget awaiting action on the floor of the Republican Senate that, because the Budget Committee could not produce a majority for anything else, gives Reagan little more for defense than did the House, rejects most of his latest proposed cuts for social programs and echoes the House on taxes.

Senate Budget Committee Chairman Pete V. Domenici (R-N.M.), joined by other Republican leaders, has said he will work with the White House on amendments to safeguard Reagan's tax program and boost defense spending. And some House members are reportedly becoming a little squeamish about their defense and tax numbers, which they thought the Senate would surely modify.

But members brave enough to predict a final budget outcome are nonetheless anticipating a slowdown in the defense buildup, a somewhat looser rein on domestic spending and at least some new taxes--perhaps the $10 billion to $15 billion contemplated by House Ways and Means Chairman Dan Rostenkowski (D-Ill.)--to help contain the deficit.

Although this would not unravel Reagan's program, it certainly would fray the edges.

The budget process, adopted nearly a decade ago as part of a compromise to make President Nixon spend what Congress had appropriated, has become a time-consuming, mind-numbing, numbers-crunching monster to many members who complain that it distracts lawmakers from more important business. Hardly a month goes by without an obituary for the process, and it always faces yet another critical test, as the revival accounts usually put it.

But nothing beautifies the beast like an assault from the executive branch, as Weinberger discovered when he suggested that Reagan would be better off if he tried to skirt the budget process and deal in the old way with the two houses' Appropriations committees.

Even Democrats who had a nasty word or two for the process when it was being used to gut their favorite programs rallied to its defense.

Moreover, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) indicated the price the administration might have to pay for no budget when he declared before the Senate committee vote Thursday that, if Congress does not wrap up a budget quickly, the House will start cranking out appropriations bills based on the House-approved budget.

This would mean $30 billion more than Reagan wants for domestic spending and an after-inflation military spending growth of 4 percent--the administration says it is really only 2.3 percent--instead of the 10 percent sought by Reagan.

Moreover, there is no guarantee the appropriations process will be more generous to the Pentagon than is the budget.

In fact, figures compiled by Rep. Les Aspin (D-Wis.), a member of the Budget and Armed Services committees, show that Reagan's first budget allotted the Pentagon $208.7 billion for fiscal 1982, or about $9 billion more than was finally appropriated. The fiscal 1983 budget allotted $238.5 billion, about $7 billion more than was appropriated.

In addition, as White House chief of congressional relations Kenneth M. Duberstein pointed out last week, it is difficult if not impossible to force cuts in major benefit or entitlement programs except through the reconciliation powers under the budget act.

With Congress at a pivotal point in deciding how far to stray from the Reagan path, Sen. Slade Gorton (R-Wash.) may have summed up the three Reagan years most succinctly Thursday as he voted in committee for the Democrats' tax plan simply to keep the budget process on track.

If 1981 was president's year and 1982 was the Republican Senate's year, Gorton said, "1983 right now looks to me like the year of living dangerously."