The administration's budget is no longer the only federal spending and taxing plan in town.

Although attention has tended to focus on the budget porposals made by the President - be it Carter, or Ford before him - the government today is operating under a spending and revenue plan devised by Congress. In fact, while a presidential budget is an important statement of the priorities and economic policy of the executive branch and therefore a prod of incalculable dimension to Congress, the president by himself never has been able to appropriate a dollar or tax a citizen.

That power has been reserved to Congress since the start of the Republic. But until now, Congress has been at a terrible disadvantage when trying to understand the overall impact of its spending and taxing decisions. The White House alone had the machinery to porpose programs, monitor spending and decide how much of society's resources it wanted to devote to federal programs.

Congress never before has been equipped to measure one program against another in terms of some sort of overall goals set out as spending ceilings and revenue floors. Now it is, and those spending and revenue levels it set in the formal budget it adopted in the fall are binding on legislators.

As a result, the House and Senate must formally revise the spending and revenue totals they adopted last September to accommodate an economic stimulus package - their own or the one proposed by President Carter last month. The need for tax and spending changes to cut unemployment and boost economic growth has been accepted by the administration and a majority of Congress. But the House and Senate cannot take floor action on most of the stimulus proposals until Congress adopts a revised fiscal 1977 budget.

The congressional budget process is in its infancy. After a trial run in fiscal 1976. Congress put together its first "real" budget - in which it forced itself to deal with overall spending and taxing rather than with each tax or spending bill separately - for the current fiscal year.

Congress adopted the process both for self-discipline and give it to the tools to deal with the president on an equal basis in spending and policy matters.

Nearly all spending programs look good in isolation. But when they are totalled up, they may far exceed the overall level of federal spending desired by Congress. The legislature not only set overall spending ceilings for itself now (a spending bill which would cause Congress to exceed the ceiling is out of order), but the budget committees do a continuous policing job, telling legislators what passage of a particular bill will do to other established priorities.

To give it the the ability to deal on an equal footing with the administration - to devise its own budget rather than react in one way or another to a set of spending and taxing priorities devised by the executive branch - Congress established two policy-making committees (one in the House, the other in the Senate) with dozens of experts, as well as a joint Congressional Budget Office, headed by respected economist Alice Rivlin.

The Congressional Budget Office was conceived of as the congressional answer to the administration's Office of Management and Budget, which is directed by former Atlanta banker Bert Lance. The Congressional Budget Office is sort of a technical adviser to the policy committees - keeping track of the numbers and outlining budget alternatives without taking positions.

But a budget is more than a statement of spending and taxing. It is a major tool of economic policy. When the economy is lagging, as the administration and Congress judge it to be today, the budget can be used to substitute public demand for private demand by running a substantial deficit. When demand is heavy and inflation threatens, fiscal policy can tighten up and the budget can run a surplus.

The budget process gives Congress the mechanism to judge the overall fiscal policy effects of a program proposed by the White House and the ability to devise its own fiscal policy.

In the past, as one Hill staffer noted, a president would make his fiscal policy proposals - Carter's program to stimulate the economy calls for $13.8 billion in tax reductions as well as $2.2 billion in spending increases in the current fiscal year - and the tax-writing committees would deal with the revenue portions and appropriations committees would work on the spending proposals. Until creation of the budget process, Congress was unequipped to deal with the package as a whole, however.

The Senate and House budget committees, headed by Sen. Edmund S. Muskie (D-Maine) and Rep. Robert N. Giaimo (D-Conn.), judged the Carter package to be too small on the spending side. While the President asked initially for $1.7 billion in spending programs - and has revised his request up to $2.2 billion - the Senate has voted a spending package of $3.4 billion and the House wants to spend $3.7 billion. Members of the House and Senate budget committees will meet in conference Monday afternoon to iron out the differences in their budgets.

The new budget process can cause some confusion. At some time, such as now, there can be as many as four federal budgets on the table for the same fiscal year. For example, for fiscal 1977 alone, there is an amended budget proposed by Carter last month, a budget approved by Congress last fall which is still binding on the legislature, and two pending revisions to that budget, one approved Tuesday by the Senate and the other approved Wednesday by the House.

Even before Congress completes action on its amended 1977 budget, it will have to begin work on the budget for fiscal 1978. In one of his final acts as president, Gerald Ford proposed a budget for federal spending year 1978 (which begins Oct. 1, 1977), and last Tuesday President Carter sent to Congress his amendments to the Ford budget restoring many of the spending cuts Ford proposed and adding the second year of his two-year stimulus package.

By May 15, Congress must come up with a working budget (called the first concurrent resolution) and by Sept. 15, two weeks before the start of the fiscal year, must pass its second concurrent resolution which sets binding spending ceilings and revenue floors. The budget committees act in consultation with the various appropriations committees.

The budget resolutions set overall levels for spending, revenues, the public debt and the deficit, as well as the authority to commit the government to spend money either in the current fiscal year or beyong. They also contain detailed outlay and budget authority estimates for 17 different federal functions - such as defense and income security - but the only binding totals are the aggregate ones.

So a congreesional budget formulated with the idea of, say, spending $100 billion on defense and $315 billion on other programs such as income maintenance, is not violated by actions which boost defense spending to $110 billion so long as other spending programs are reduced by $10 billion. But a spending bill which would raise outlays above $415 billion is out of order under the rules because it would violate the overall ceiling.

Many congressional observers have worried that the very discipline the budget process is designed to inject could spell the death of the process. Congress has tried to budget more rigorously before - most recently just after World War II - only to see the process founder when the body was forced to judge between two or more attractive spending programs.

Indeed, some observers see this year's decision to write a third budget resolution - which amends the "binding" ceiling adopted last fall - as a breakdown of discipline. House committee chairman Giaimo disagrees.In a recent interview, he said he is aware that Congress might be tempted each year to reopen previous decisions by writing a third resolution.

Giaimo argues, however, that the current economic picture is such that a third resolution is required.

But the New Haven Democrat maintains that revisions of final budgets must be rare if the budget process is to have any meaning. Muskie, his Senate counterpart, claims that if there were no provision to amend the budget this year, the budget process would have broken down because it is clear that Congress must take steps to stimulate the economy.

The congressional budget can differ sharply from the president's. A president can veto appropriations bills and, with very circumscribed authority, can choose not to spend appropriated money (although he can be forced to). For the most part, however, he must work within the budget range adopted by Congress.

Nevertheless, the presidential budget remains an important document. It provides masses of data, sets forth the spending and taxing priorities of an equal branch of the government and provides a benchmark to which congressional budget makers refer in constructing their own document.

And despite their seeming precision, budgets are very inexact documents. Differences ranging into the billions of dollars may be due to technical factors rather than policy divergence. If a congressional budget maker thinks a billion-dollar defense payout will be due in fiscal 1977, he will put it in his outlay total for that year. A presidential budget maker might realistically believe the payout will happen on Oct. 2, and not make provision until fiscal 1978. Their respective budgets will differ by $1 billion in outlays.

Or fewer people may claim food stamps or unemployment insurance than originally was thought, which would lower outlays below projections.

Budgets, presidential or congressional are high-level, expert guesses. Even if they are off by 1 per cent, a trivial error as forecasts go, that is $4 to $5 billion at current spending levels, hardly a trivial sum.