It is plain now that the opposition to President Reagan and his program is beginning to find its voice. The 260,000 people who assembled on the Mall last Saturday at the call of the AFL-CIO and some 200 other organizations to protest the Reagan economic policies was the largest such demonstration since Vietnam War days.

This weekend, the Democratic Party will hold its first major training session for the 1982 campaign, in Des Moines, and on Oct. 1 it will parade a number of mayors before the microphones at a dinner here to describe the damage they say will be done by the Reagan budget cuts that go into effect that day.

Meantime, House Democrats have recalled that the committees they control are allowed to conduct investigations, and Tip O'Neill has launched a number of them into regional hearings focused on the effects of high interest rates and scarce federal dollars.

As a result of all this, the Republicans are getting shaky about their support of the new round of budget cuts. And Washington, a city whose inbred discussions produce violent swings of opinion, has--in its typical fashion-- gone from thinking that Reagan is king of the world to thinking he is a political fall guy.

What everyone needs to do is step back one pace and take a deep breath. Otherwise, we are about to jitter ourselves into serious trouble.

We have been down this road before --exactly 12 years ago. Then, the Republican president with nine months in office was Richard Nixon, and the issue that brought thousands to the streets was Vietnam.

The troubles in today's economy are, thank goodness, a lot less ugly a mess than Vietnam was 12 years ago. But there are certain similarities in the situation. The basic problem in both instances is one the Republican administration inherited from its Democratic predecessor. "Curing" the problem is the basic mandate each Republican president received from the voters.

In both instances, the Republican president put in place by the fall of his first year a long-term strategy for extricating the country from its bind. And in both instances, the opposition has gone to the streets with the claim that the program is not really as advertised in the previous campaign and, even if it is, it is not producing results as fast as they are needed.

There was plenty to criticize in Nixon's Vietnamization, and there is plenty to doubt about Reaganomics. But it seems to me that any fair-minded appraisal has to conclude that there is greater political legitimacy to Reagan's current effort than there was to Vietnamization, and therefore a more compelling case for caution in condemning it.

While lives are being hurt by the Reagan economies, the human damage cannot be compared with that which resulted from Nixon's decision to attempt a gradual pullout, which prolonged the agony of the Vietnam War.

The Reagan plan--to a much greater extent than Nixon's--was suggested in fairly explicit terms by the president's campaign statements. True, he dodged the painful truth about reductions in entitlement programs and the shift of responsibilities to state and local governments. But anyone who did not understand that Reagan was proposing a major trade-off--lower taxes for fewer federal government services--was not listening.

But the most significant difference is that Reagan's plan has been given explicit approval by Congress, while Nixon's represented purely executive- branch decision-making.

Moreover, it was given approval by Congress as a long-term policy, not a quick-fix expedient.

As readers of this column know, there have been grave doubts expressed here about the pace and scale of the reduction in federal responsibilities and the manner in which programs have been handed off to states and cities, or just abandoned. I have been even more skeptical about the size of the tax cuts, and the promise of future tax indexation is one I thought no prudent Congress should make three years in advance.

But this policy was approved by majorities less than two months ago. It has not yet been put in place. To consider scrubbing it now--or replacing it with an invisible alternative--strikes me, not as a sensible political judgment, but as a reaction of pure panic.

There will be time--and need--for mid-course corrections. But to attempt them in the waning days of a congressional session, rather than in the 1982 consideration of the Reagan budget, entails even greater risks than the gamble implicit in Reaganomics.