PRESIDENT REAGAN has now embarked upon an extremely daring venture. He is setting out to bend and turn the fiscal trends that the last three presidents have bitterly denounced but finally judged uncontrollable. Mr. Reagan presents his tax and budget cuts as a program of reform for the full four years of his term, and he is inviting the country to judge him by it. There's a spirit of audacity to it -- like Babe Ruth's pointing to the center-field stands as he stepped up to the plate.

End those federally funded public service jobs -- all of them? Reduce those subsidies for the dairy cows in Wisconsin, and for the foreign sales of Boeing jetliners? Make the yachtsmen pay their share for the Coast Guard? Recent political history is full of the sad stories of failed politicians who could give Mr. Reagan a hundred reasons why it won't work. But perhaps it will work.

The test is not going to be the response of the economic indicators alone. It will be the change in Americans' attitudes in general toward this experiment, now and over the next four years.It will be the change in their sense that the economy is out of control, and that the government has become not a servant but an adversary. It is possible to believe, as we do, that Mr. Reagan has been some bad choices along with the good ones in this program -- and yet to acknowledge a strong national interest in its success.

As Mr. Reagan pleasantly observed in his address to Congress last night, the people who oppose this cut or that one now have a responsibility to come up with something better. If you don't like, for example, the reduction in aid to college students, what would you prefer to abolish? Or are you prepared to argue that the present deficit and the present inflation are preferable to any alternative? If all goes well, after two or three years of intense controversy the country will have worked out a surer and more defensible definition of federal responsibilities -- which in turn will provide a solid political base for the taxes to pay for them.

Regarding Mr. Reagan's tax plan, the administration would be well advised to warn average middleclass taxpayers that they will be paying more, not less, to the federal government in 1981 than last year. Inflation will ensure that the typical taxpayer will pay not only more dollars but a higher proportion of income, even if the Reagan bill is enacted as proposed. The administration also intends to make the tax system somewhat less progressive, with the biggest cuts in the upper income ranges. It has a defensible case for this policy, which makes it doubly foolish to suggest, a bit disingenously, that the cuts are uniform for all income levels.

But the great and central risk is, of course, that the inflation rate might not decline as predicted and that Americans do not see the economy gaining stability. Mr. Reagan has attributed the inflation wholly to federal deficit spending, but under his plan the deficit will not come down significantly until 1983. Not even the administration sees any very early or rapid drop in the inflation rate, and the administration's strategy makes no allowance for the kind of worldwide upheaval -- oil crises, food shortages, currency devaluations -- that undid his predecessors' planning.

The purpose of this assault on the budget and the deficit goes well beyond economic policy, and Mr. Reagan understands it perfectly. What most of the voters seem to want, above all else, is a demonstration that government can actually be managed by the people elected to do it. Each of the last three presidents told them that the federal deficit is highly inflationary, and that inflation is a menace. Each of these presidents left office with either the deficit, or the inflation, or both, worse than he found it. As people watched these failures, further questions legitimately formed in their minds. If one administration after another proved unable to deal with its own household finances, its incessantly proclaimed top priority, was its competence to be trusted in the less visible and comprehensible matters of defense, strategic arms, diplomacy or finance?

Mr. Reagan has caught that point precisely. "We are in control here," he said last night. "There is nothing wrong with America that we can't fix. "But it will take more than a speech to persuade people. The long inflation has eroded more than the value of the currency. There is far more than economic performance at stake in President Reagan's struggle to control, at last, the budget.