On Wall Street, the Reagan administration now has zero credibility: when the White House passed the word last week that, yes, the economy is sluggish, but it still sees a boom-like 4.5 percent economic growth in 1987, you could hear the snort of disbelief above the sound of stock tickers.

Just like, as Barron's neatly put it, the old Brooklyn Dodgers' routine: when they lost the pennant race, as they usually did year after year, the Dodgers' management would intone: ''Wait till next year!''

Says former Office of Management and Budget aide Lawrence Kudlow, now chief economist for Bear Stearns: ''As a former spear-carrier myself, I regard the White House forecast as beyond even a leap of faith'' from the dismal 1.1 percent growth in the second quarter of this year. Kudlow predicts that when OMB issues its mid-year revision on Aug. 4, the one certainty is that the business and financial community will not believe the numbers, whatever they are.

For example, the word on Capitol Hill is that the OMB, to keep the 1987 budget-deficit figure from looking even worse than it may be, plans to use a fairly high inflation projection for 1987. This would tend to exaggerate revenue prospects.

Instead of a candid reappraisal of prospects, which would acknowledge the failure of the economy to respond to traditional expansionary elements -- such as lower oil prices and declining interest rates, the Reagan administration keeps promising that everything is going to turn out all right. ''Some of the slower growth we saw last quarter was actually the result of some very good news on the horizon, and that's the tax-reform bill,'' President Reagan said with a straight face in his Saturday radio chat.

Once the bill is passed, Reagan predicted, ''we can expect business to really start moving.''

Maybe it will. But the business community doesn't believe it for one minute. True, tax reform is overdue, and in the long run should be a plus for the nation. But as Sam Nakagama of Nakagama & Wallace, Inc., told his clients, tax reform in the short run deepens the recessionary elements in the economy. For example, it reduces incentives for capital investment. It also will have a major negative impact on commercial real estate, especially in the Southwest, compounding problems for banks already suffering from shaky oil loans.

Moreover, if the highly touted 27 percent top income-tax rate doesn't go into effect until July 1, 1987, while loopholes are closed Jan. 1, 1987 -- the present plan -- then the nation is confronted with the equivalent of a 1987 tax increase. That will dampen consumer spending, currently the main prop under the economy.

What it adds up to is a lot of doubt about the immediate future. As Federal Reserve Board Governor Wayne Angell told me: ''There is no hard evidence that the economy is on a higher growth path'' than the 2.5 percent first-half average -- certainly not the 4 percent the administration sticks to for the remainder of 1986. Angell is not predicting a termination of the current long expansion -- simply a continuation of the presently uninspired pace.

Does it makes sense, some now ask, to pursue the Gramm-Rudman targets with all vigor, at a time that the economy is weakening?

Former Economic Council Chairman Charles Schultze says Congress should still pursue the goal of deficit reduction, but with the effective date of an ultimately necessary tax increase pushed into the future. In an ideal world, it would have made better sense, Schultze told me, if the Gramm-Rudman deficit targets had been tied to changes in the GNP growth rate.

Kudlow would like to link cutting the budget deficit to a further reduction of interest rates by the Federal Reserve Board. Paul Volcker, Kudlow suggests, could state that he looks favorably on lowering interest rates if Congress meets that 1987 Gramm-Rudman target.

These suggestions point up the dilemma of the moment: it has become apparent that chances of resolving the budget-deficit problem are fading.

When the administration's original Rosy Scenario went down the drain, it also took a toll of potential Treasury revenues. On Capitol Hill, experts figure the drop in growth estimates will cost about $25 billion in 1987 revenues. Thus, the $144 billion Gramm-Rudman deficit target for fiscal 1987 is now probably unreachable without Draconian budget slashes or a quick tax hike. (The Congressional Budget Office may use a figure in the neighborhood of $170 billion in its revisions next week.)

If I were a Republican running for reelection to Congress this November, I would wonder if the administration's latest snow job could backfire. In his radio chat, the president berated the ''doom and gloom artists who . . . have been painting dark pictures.'' Pay no attention, he said, ''America's economy is strong; our future bright. . . . 'You ain't seen nothing yet.' '' The true believers make up a short list, headed by Nancy Reagan and Economic Council Chairman Beryl Sprinkel -- and I'm not sure about Mrs. Reagan.