The loudest American cheering over Margaret Thatcher's "landslide" victory is coming from the wrong quarters for the wrong reasons--from those who make no distinction between the effect of her reelection and its cause.

The effect is worth at least a deep sigh of relief for those engaged in the making of U.S. foreign policy. By nearly destroying the Labor Party, Thatcher has wiped out, for five years presumably, the threat to U.S. interests from Labor's far-out foreign policy doctrines: the closing down of U.S. military bases in Britain; the reneging on the British end of the NATO deal calling for deployment of American medium-range nuclear weapons in Europe by the end of the year, absent an agreement with the Soviet Union that would obviate the need.

But what of the causes of Thatcher's triumph? What of the contention that she has demonstrated something wonderfully cheerful to political strategists for a Laffer-curver, supply-sider, American president--that conservatives can win in hard times? If unserving devotion to austerity, and a sparkling performance in controlling inflation can prevail in Britain over near-depression unemployment levels and a soaring bankruptcy rate, why not in the United States?

Because the Anglo-American analogue crumbles almost at first touch. Where Ronald Reagan disarms his opposition with good cheer and laid-back charm, Thatcher disarms hers with a schoolteacher's discipline and cold, calculated resolve. Where Reagan lays about him at big government almost indiscriminately, Thatcher is wisely selective, knowing full well that pension programs and jobless benefits put in place in Labor's halcyon days are sacrosanct.

Without the socialist cushion, even today's Labor Party might have been able to capitalize on the consequences of Thatcher's economic ideology--if it had not carried "loyal opposition" to the point of political suicide. First there was the Labor split after the 1979 loss to Thatcher, the formation of the splinter Social Democrats, and their hook-up in the alliance with the Liberals. Then there was Labor's turn to aging, diehard, left-wing leadership; Michael Foot's ineptitude was best measured by a poll showing that with deputy party leader Dennis Healey in the top job, Labor and the Conservatives would have run neck and neck.

And then there was the Falklands war, with Thatcher playing Queen Victoria to a populace still asking, deep-down, for the long-gone days of the empire. Having stumbled into the conflict (clumsily enough to cause her foreign minister to resign) she emerged from it after a heavy loss of life on both sides (and no permanent resolution of the issue) as a can-do lady, in sharp contrast to the can't-do-anything-right image of Labor. In precisely this sense, last week's vote was not so much an expression of wide public confidence in Thatcher as it was a vote of no confidence in Labor.

To translate all this into anything meaningful for America you would have to believe that a U.S. president can call an election when it bests suits him and, with 42 percent of the vote, not only win the White House but gain absolute mastery over both houses of Congress; that Democratic moderates" will split off and join, well, John Anderson, while the traditional party nominated, let's say, a reincarnated Henry Wallace; that a seemingly senseless, small but bloody war would do for Ronald Reagan in post-Vietnam America what one did for Margaret Thatcher.