When the Senate passed a deficit- reducing budget resolution at 3 a.m. last Friday, it seemed the most constructive step yet taken by either branch of Congress to halt the flow of red ink that threatens to destroy the economy.

The 50-49 vote -- Vice President Bush casting the tie-breaker -- offered the prospect of a $56 billion reduction in the fiscal 1986 deficit, and $295 billion over the next three years. It was interpreted as bullish by financial markets, which assumed the burden had been placed on the Democratic-controlled House to match the Senate savings.

But there is considerably less here than meets the eye, and some of the cheers voiced by those worried about the economic consequences of the swollen deficit are, at best, premature.

The real value of the Senate package, in terms of reducing the deficit from fiscal 1985 to fiscal 1986, is more in the range of $20 billion to $30 billion, rather than $56 billion. That's better than nothing. But it will leave the deficit still at a dangerous $180 billion mark, according to New York economist Henry Kaufman and other experts.

Most of the newspaper and TV news reports on the Senate budget package refer to a "freeze" on defense spending, after allowing for inflation. But the freeze applies only to new authority, not to actual outlays, which will rise (under the Senate bill) by about 8 percent, or $18 billion in 1986.

Actual spending in 1986 will still be on the rise because funds are paid out from prior-year appropriations. There is a ton of money socked away in the pipeline since Reagan and Congress began the huge defense buildup in 1981 that will finance annual outlay increases for years to come.

Ironically, the only real freeze applied by the Senate is on cost-of-living adjustments for Social Security recipients, who thus become scapegoats for spending excesses elsewhere. Social Security expenditures are more than covered by Social Security tax receipts. And the Senate package, as the Democrats charge, breaks Reagan's 1984 campaign promise not to reduce Social Security payments, in order to rack up a $6 billion saving.

Even if the freeze on defense spending is illusory, it was enough to raise the blood pressure over at The Wall Street Journal, where the editorial board, along with contributor Irving Kristol, couldn't hide its sense of shock. After all, Reagan had initially asked for a real increase of 5.9 percent in new 1986 money for the Pentagon, the same as in each of the three prior years. He retreated when the Senate balked and tried to get a 3 percent boost. When Majority Leader Robert Dole and other GOP leaders stood firm against that, he warned that zero growth would be "irresponsible." But he finally caved in, as the Journal and Kristol see it.

"The more we watch this process, the more we think Washington is overlooking one other significant way to save time and money: Abolish the White House." So said the Journal on May 15. In the same issue, Kristol mourned that supply-sider Reagan had weakly succumbed to "the intellectual and political pressures of the traditional conservatives" in the government who see economic disaster unless the deficit is sharply reduced.

Sourly, Kristol chalked up the White House budget "fumble" and two others -- Bitburg and defeat of military aid to Nicaragua -- to the fact that "the Reagan administration does not understand its own ideology all that well, and does not know how to articulate it."

The Journal and Kristol ignore a basic reality: Reagan is a lame-duck president whose influence and magic are beginning to wane. Yet, by yielding to the better judgment of establishment Republicans, Reagan has forced House Democrats to share in the difficult deficit-reducing decisions.

From the Reagan perspective, Dole & Co. salvaged a cut in the fiscal 1986 deficit. Whether it's $20 billion or $56 billion, it's something. And so far, Reagan has avoided a tax increase. But like most successful politicians, Reagan is a compromiser. That's the real definition of "Reagan being Reagan." How quickly one forgets that Reagan, for all his anti- tax rhetoric, signed three revenue- boosting bills in the past four years.

A trusted presidential adviser says: "The president has told me any number of times, 'If I can't get a 100 percent of what I want, I'll take 80 percent or I'll take 90 percent or I'll take 75 percent.' It's important to the institution of the presidency that he be seen as succeeding." Reagan, in other words, won't be Kristol.