A general consensus in the country favors beefing up national defenses at this time. But the consensus is weak and subject to rapid change.

Sure evidence of the fragility comes in a series of articles asserting that the military spending will plunge the country into ruinous inflation. While that charge does not survive serious analysis, the mere staking of the claim demonstrates the need to design now, while the mood is favorable, a defense program that has staying power for the future.

Probably the best, and certainly the most influential, of the recent articles was published by Lester Thurow, a professor of economics, in the May 14 issue of the New York Review of Books. Thurow's argument is well expressed in the title of his article -- "How Reagan Can Wreck the Economy." As it happens, the argument and the peril it invokes rest on a false comparison and two unfounded assumptions.

The false comparison is between the present and the Vietnam era. Thurow writes:

"President Johnson . . . wanted both the Great Society and the war. But if he wanted to have both and not wreck the economy, his only option was to raise taxes sharply. He chose not to do so, and he wrecked the economy.

"President Reagan wants both dramatic tax cuts to encourage investment and an even more extensive military buildup. But he cannot have both without wrecking the economy further unless he is willing to raise taxes drastically on private consumption. He has chosen not to do so. If his current program is carried out, he too will wreck the economy."

In fact, the similarity between what Johnson did and what Reagan proposes is purely rhetorical. Johnson pushed the country into spending for both guns and welfare. Reagan is calling for major cuts in social programs. Thurow doesn't like the cuts, and neither do I. But in approach, the two presidents are worlds apart.

As to unfounded assumptions, one is that the Reagan defense program is already known. Thurow, for example, foresees huge and very rapid commitments to development of sophisticated weapons systems. He asserts there will be "shortages of . . . craftsmen, engineers, and scientists. Such people will be attracted to military production. Defense contractors will entice workers away from civilian firms by paying higher salaries as they up their work force on a crash basis." The more so, according to Thurow, because the military work will be "closer to the frontiers of scientific knowledge." No doubt there could be a squeeze on skilled manpower -- and other items, too. Bottlenecks might develop if the Reagan administration went for crash programs to produce a new bomber, or to raise the Navy to 700 ships, including 15 carrier groups, or to develop a fancy new basing system for the MX missile.

But so far, crash programs to build such weapons are only gleams in the eyes of defense zealots. The actual choices have not been made. Thurow is simply imagining what will happen. As yet there really is no Reagan defense program.

The second false assumption is that the Reagan economic program cannot possibly work. The administration claims that its program will foster a burst of investment with big gains in productivity. Thurow flatly denies these possibilities. For example, he writes that the "Reagan administration assumes that productivity is going to return to a 3 percent rate of growth almost instantly, but . . . such an increase in productivity has never happened before in our history."

In fact, it has. The official estimate for the first quarter of the present year figures a productivity rise of 3.9 percent. While that may not be sustainable, the truth is that nobody knows.

To be sure, the economic climate is distinctly chancy, and hard choices in defense have not yet been made. Moreover, there remains in the country a powerful residue of the dovish, anti-defense sentiment fostered by the Vietnam War.

But the important point, in these conditions, is the development of a defense program that can survive the troubles ahead. That means a stress on the skilled manpower and high readiness that are immediately essential in such danger spots as the Persian Gulf. It means avoiding crash programs that are intrinsically wasteful. It means emphasizing a long, slow buildup with a momentum that carries beyond the mood of the moment. It means, above all, something we do not now see at the Pentagon -- a capacity to frame, by sharply focused analysis, a strategy for defense.