President Carter sends his fiscal 1979 budget to Congress today, but the budget you see is not the budget you'll get.

Before the spending plan for the year beginning Oct. 1 can become law, it must go through the congressional budget process that Congress set up in 1974.

That process was intended to allow - and force - Congress to do something it had never done. It had to decide in the aggregate each year how much the government ought to tax and spend, and how large the deficit should be. Then it had to live within those limits.

In the first year the process was tried - 1975 - James L. Buckley (R-Con-N.Y.) was a member of the Senate and its Budget Committee. He filed a dissenting minority report when the committee reported out its spending recommendations: they were too high for him.

Buckley expressed his skepticism about the new prodedures in Latin. "Natura non facit saltum," he wrote at the start of his dissent - "Nature makes no great leaps." He doubted that a simple change in the rules could cause Congress to change its ingrained spending habits.

That same source of disappointment continues to dog the process. House Budget Committe Chairman Robert Giaimo (D-Conn) said recently, "There is a widespread feeling right now, which I share, that the congressional budget process is in trouble. Congress has followed the new process quite faithfully. . . However, on substantive matters. . . Congress is too willing to negate the restraint implicit in the process whenever it is politically appealing."

In other words, Congress quickly learned to bend the new rules to its will and spend as before.

"Discipline on tax and spending matters cannot be legislated." Giaimo concluded. "Congress must discipline itself and decide whether it will take the process seriously."

Senate Budget Committee Chairman Edmund S. Muskie (D-Maine) made similar remarks at recent hearings on the budget process. "During the past year we have noticed two disturbing trends: first, the threatened resurgence of the free-spending mentality even in the face of our worst peace-time dilicits," he said.

"The second trend is a growing resistence and unfortunately resenment, on the part of certain committees and an apparent contempt on the part of some members for adhering to the clear requirements of the Budget Act itself."

Both Muskie and Giaimo feel this is a crucial year for the budget process.

The way of the system works, each spring the Budget committe propose congressional tax and spending targets for the fiscal year ahead. Congress then amends and approves the amounts, first in May and then in "final" form in September.

If Congress decides to spend more than allowed in its budget for one thing, it must then vote to spend less on some other, or else expressly approve the higher deficit its spending decision has produced.

On several occasions in the few years they have been functioning, the Budget committees have beaten back efforts to exceed congressional spending targets. On several other occasions, however - last year's farm bill is the most notable recent example - they have failed.

This year the budget process will face even greater tests, because the proposed $24.5 billion net tax reduction will increase the pressure to hold down spending.

In addition the Democratic controlled committees may have to pit themselves against a Democratic presedent, sorely testing Congress' loyalties. Carter, for instance, is expected to ask for a real increase in defense spending and both BudgetCommittees traditionally have cut defense spending. They may also quarrel with him on the amount of the tax cut. he has proposed.

The committees have already been accused of merely compiling what other committees want to spend. "As it has emerged, the budget process has not set directions in fiscal policy." Rep. Barber Conable (R.N.Y.) says. "It has tended to add up revenues and expenditures and say "Oh, isn't that interesting, a $60 billion deficit."

There is, however, another school of thought in all this, one which holds that the process ought not be judged so much by whether it succeeds in restraining spending as by whether it regularizes the spending process on Capitol Hill and makes the legislators more accountable for their actions.

Indeed, some in this school think it is a good thing that Congress has sometimes voted to exceed presidential spending recommendations, as it did under President Ford, for example. They say the economy needed the extra spending to pull it out of recession.

They also say the process - and the experts who have come with it on the staffs on the two Budget committees and the Congressional Budget Office - have added immeasurably to the sophistication of the anual spending debate on the hill.

"You just can't underestimate how Congress as an institution has advanced in its economic thinking as a result of the Budget Act," Senate Budget Committee staff director John McEvoy said. "I am convinced the 1960s was the era of the lawyer around here, with civil rights and like legislation being in the forefront. But the 1970s is the era of the economist. Because of the budget process, senators who barely understood tax and spending terms are now talking about things like the "reflow effects of taxation," referring to the way in which a tax cut can stimulate business and as a result bring in more taxes.

Congress once before-in the postwar 1940s-adopted a new budget process like the current one to the discipline itself, but then abandoned it after a few years. Most odds-makers think this one will be different, and will survive. The question is whether it will survive as a score-keeping system or as a power to be reckoned with.