Tonight, assuming President Reagan sticks to the State of the Union message he was to have given the day of the space-shuttle tragedy, his theme (in the words of White House Chief of Staff Donald Regan) will be: "America is not only back -- America is looking forward and is going forward."

That's a beautiful theme, nicely suited to Ronald Reagan's enormous talent for making beautiful themes believable. But unless you believe (which few people do) that the Gramm-Rudman budget-balancing law is going to be (a)found unconstitutional, (b)repealed or amended, or (c)ignored by the president if push comes to shove, it is also a theme out of touch with domestic political reality. It squares as badly with the administration's own estimate of the state of American security.

Under the Draconian constraints of Gramm-Rudman, America is "going forward" -- rather like the Light Brigade -- into the teeth of the most crunching battle of the budget in memory. Because the bargaining will be so intense and the outcome so uncertain, the economic and social consequences are beyond prediction. There is, however, no way to escape the implications for national security and the effective exercise of U.S. influence in the world.

By the very nature of its unbending insistence that defense spending is untouchable, the administration is inviting a dangerous misperception of American weakness, because defense spending is going to be more than just touched. It is inviting the impression of an America that is "back" only in the most ironic sense: after one of the biggest defense- spending binges in history, "America is back" pretty much where the buildup began five years ago, with an administration talking a losing game of catch-up with the Soviet Union.

I say "losing" not because Defense Secretary Weinberger puts it that way. Defense secretaries traditionally talk in apocalyptic terms about what will happen if we do not spend what they want for defense. But with Weinberger, there is a significant difference.

By a wide margin, Weinberger outdoes just about every predecessor I can think of in his insistence on defense spending at levels that the public and its representatives in Congress have shown themselves increasingly unwilling to provide. Weinberger may be right about the threat and what is needed to meet it. That only reinforces the point. The current condition of the defense budget is almost a perfect example of how easy it is to squander a clear public mandate for steady growth in defense outlays by overstating the case.

The numbers speak for themselves. The Reagan administration has undeniably strengthened U.S. defenses by five years of real growth averaging 8 percent annually. But then came what even its authors call an "act of desperation" in the form of Gramm-Rudman. The result is zero-percent real growth in defense spending in the current fiscal year.

That is not "going forward." It sends exactly the wrong signal to adversaries and allies alike, especially those in Europe who have committed themselves to join in a concerted NATO policy of increasing defense spending at the annual rate of 3 percent, after inflation.

Still less do you get a sense of America "going forward" when you listen to Weinberger hold forth on future prospects -- on TV talk shows, at a breakfast meeting with supporters, and again at lunch in his Pentagon office. He speaks of the "extraordinary vices" of Gramm- Rudman, a reference to the sequestering mechanisms that would trigger indiscriminate, across-the-board cuts in everything, including defense, if next year's budget deficit exceeds $144 billion. And here, few would quarrel with Weinberger's warning that the results would be "extremely damaging to national security." That's the whole idea: to bludgeon Congress and the executive branch into budgetary discipline.

The trouble is that Congress is not going to accept the alternative that Weinberger insists it must: adopt the president's budget as it is, with all the cuts on the domestic side and without laying a finger on defense. If the president abides by his solemn vow not to raise taxes, something in the proposed 3 percent real growth in defense spending in his new budget will have to give. Yet Weinberger insists that even this growth rate is already "way under what we need."

That sort of scare talk served Reagan well when he rode to victory in 1980 with a massive defense buildup as a centerpiece in his campaign. There was strong popular support for defense.

But a recent New York Times-CBS poll confirms that the consensus is long gone. In 1981, 62 percent of the public wanted higher defense spending; only 7 percent favored decrease. Today, only 17 percent want more defense spending, 26 percent want less and 53 percent of the public would hold the line at present levels.

Now, you can blame the public or Congress for having the wrong priorities. But you cannot derive from those figures a sense of "America going forward" in anything like the way that the president's secretary of defense thinks is vital to American security.