It is the night Ronald Reagan has been waiting for, the moment he has been anticipating in nearly 30 years as banquet speaker, conservative folk hero, governor of California and candidate for president.

Tonight is the night St. George slays the dragon and David downs Goliath. Tonight is the night the Gipper is supposed to run for the touchdown that saves the republic.

"I've been wanting to do this for a long time," Reagan said last week as he approved one of Budget Director David A. Stockman's celebrated budget cuts."

If anything, it is an understatement. Ever since he became the toast of the General Electric plant tour in the early 1950s, Reagan has been hammering away at the thesis that America's domestic ills could be blamed largely on the growth of government in what he used to call "the puzzle palaces on the Potomac."

"Most of my speeches have been on the central theme of how government growth restricts individual liberty," Reagan said a decade ago in explaining the basic message he brought to the banquet circuit.

It is a belief he holds to this day and which will invest his speech tonight outlining his proposed budget cuts to a joint session of Congress.

The irony for Reagan, in his moment of triumph, is that if he succeeds in his presidency he will be accomplishing something far different from his rhetorical "new beginning" that turns the nation in a fundamentally new direction.

"We're going to make history in that no government before has voluntarily reduced itself in size," Reagan told his Cabinet last Friday.

In fact, as the president knows and told a nationally televised audience in his Feb. 5 speech, the government is not going to be reduced in size. The fact is that the federal budget will grow from $660 billion this year to $695 billion next year to $730 billion the year after that even if -- and it is a big if -- Reagan gets the spending cuts he seeks from Congress. The fact is that the total amount of taxes the government takes will grow, too, even if tax rates decrease.

What Reagan actually is proposing is not, as his rhetoric promises, to turn the nation around, but to brake the rate of government growth. At the same time that he is promising to eliminate waste and fraud and put more money in everyone's pockets, he also is pledging to keep the safety net of New Deal programs which are an inextricable feature of the government over which President Reagan presides.

It is as if Reagan were now both David and Goliath, both St. George and the dragon of the federal government he has vowed to overcome.

Reagan has been through all this before, in California, where he served as governor of the nation's most populous state for eight years.

When he took office, he promised to "squeeze, cut and trim" the costs of government. He did slow the growth of the state work force to slightly more than the inflation rate. But he also presided over what was then the largest tax increase in California's history (a progressive one which took relatively more from the rich and corporations than from poor people) and over the growth of a government which his rhetoric had led people to think he would reduce.

The difference between the Reagan reality and the Reagan rhetoric in California was that the rhetoric was sweeping and occasionally inflammatory while the reality was sensible and incremental, with the governor usually willing to settle for far less than the loaf. It is not too much to say that in many instances he was satisfied with little more than a slice.

In relative terms, Reagan's proposals now are even more incremental. For all the controversy the cuts will cause, the near $50 billion that Reagan will seek to trim from what the Carter budget would have been amounts to less than one percent of the gross national product.

Reagan and his defenders would say that they deserve more credit than this, that they are, after all, taking a first step in a direction in which others have not dared to go.And they would point out, accurately, that the built-in opposition to any budget reductions make even small steps significant.

Certainly, the sense within the Reagan Cabinet as the big day approached resembled that attending the launching of a great adventure. Reagan was ebullient, as he quickly approved cuts here and slashes there. Near the end of the Cabinet meeting last Friday he profusely thanked Stockman for his budget cutting, saying that "while a lot of us have been sleeping at night, Dave has been out there doing these things.

"We won't leave you out there alone, Dave," Reagan said, and paused for effect. "We'll all come to the hanging."

It was the practiced humor of an actor turned politician who was about to play his dream role before a national audience. Whatever the reality, it is the performance Ronald Reagan has always wanted to give.