SO THERE'S A MAJORITY on President Reagan's side. So there's economic optimism among the American public, even those who expect to suffer personally as a result of his proposed budget slashes. So that optimism is helping the president gain the broad support he needs to win approval for his program.

That's the word from the polls, and it's not surprising. It was inevitable that the public's hope could climb both in light of the expectations of the economic miracles that Reagan is promising and because the economic hopes of the majority of Americans had reached such a nadir.

But ultimately, the people's optimism -- or pessimism -- will depend on whether his plan flies or falls, not on high hopes. After the theatrics and the applause, it's my prediction, on behalf of that part of the public that seems to be invisible in the polls and silent amidst the glowing optimism, that Reagan's plan will fail.

Reagan's economic recovery plan will fail because its underlying assumption -- that bolstering capital investment is going to be the sole resuscitator of the U.S. economy -- is wrong. It will fail because decreasing taxes to the degree that he's proposing will not create an explosion of sufficient demand and production to erase the budget deficit and at the same time it will be counter-inflationary. It will fail because the economics don't add up. It will fail because it is based on a wildly optimistic projection about production. It will fail because a budget balanced on the backs of the poor today will tomorrow pinch the wallets of those with middle incomes.

Finally, it will fail because the supply side theory -- that if you give more financial latitude to the rich, they will produce more to trickle down to the rest of us -- is a hoax.

But you've heard this before -- from such diverse voices as the NAACP, the AFL-CIO, the Black Caucus, Capitol Hill Democrats and the National Urban League. Some have produced alternative programs for economic recovery, proposed counter tax plans and noted that the Reagan administration is examining the long-term bottom line while ignoring the needs of day-to-day survival for many Americans.

As Rep. Harold Washington (D-Ill.) told budget director David Stockman at a hearing of the House Subcommittee on Manpower and Housing last week: "Mr. Stockman, sir (these policies) are exacerbating, perpetuating, incubating, fomenting revolution. These cities are going to blow up . . . ."

But all these people are getting for their trouble is the Reagan Refrain: "I pleged loyalty to only one special-interest group -- 'We the people.'" It's enough to make those who don't support the plan feel invisible. It's enough to make you want to paraphrase Sojourner Truth's lament: "Ain't I a people, too?"

But while I feel the administration has been cavalier and blind, I say enough of the lamentation. The time has come for figuring out alternatives when the Reagan balloon bursts; the time has come for figuring out coping strategies in the meantime.

This is not to say that people should give up. I'm not skipping the ground in between the proposals and the solutions once the proposals have been tried and have failed. But we need to take the long view. Although the old coalitions and the Great Society have lost out, we can't simply hunker down, we can't simply march around with picket signs.

It's clear that state and local governments are once again going to be much more important in the next few years -- assuming approval of Reagan's plan to turn responsibility for many federal programs over to the states and municipalities by handing out block grants rather than categorical ones. So, as much as I disagree with this plan, it still would behoove people to begin examing the kind of influence that can be exerted on local governing situations.

It is widely known that block grants do not bode well for blacks and minorities whose concerns have to be pitted against more powerful constituencies. But it is also true that in cities with a black majority -- cities like Atlanta, Baltimore, Gary, New Orlean and in states like Mississippi with its 40 percent black population -- the opportunity exists to exert enough influence to make sure these funds aren't totally benign to black needs.

These efforts at the local level can't entirely pick up the slack, but maybe they can lessen the blow. Howard University Law Dean Wiley Branton said this week he thinks the protection of legal and voting rights should be the key concerns. That would mean, for example, that such organizations as the Lawyers Committee for Civil Rights, the American Bar Association and the National Bar Association should try to help provide legal representation for the poor if the Legal Services Program is curtailed.

But let's face it, when Reagan asked, "Isn't it time we tried something new?" and "the people answered "yes," it was his first mandate for a return to the way it was done in the 1950s.

But the reams of statistics on the economic future that Reagan sees don't add up, and the world is very different. And while the polls looked good this week for the president, the answers for the financial future of many Americans look far less clear.