When Ronald Reagan flew east from California last Sunday, the script called for a series of talks on national security that would show him to be a sober, prudent man of peace. Instead, by a single speech, Reagan demonstrated the same capacity for irresponsibility on defense that Jimmy Carter has so painfully put on the record in 44 months as president.

In consequence, the serious defense debate the country deserves had been badly compromised. The most critical issue before the nation has become a football in a contest as to which candidate is the least competent.

Three speeches on national security were planned for Reagan's first campaign foray following the Democratic convention. First, on last Monday he was supposed to talk about peace to the Veterans of Foreign Wars in Chicago. On Wednesday, he would talk to the American Legion in Boston about the need to make the country strong. Finally, in Dallas on Friday, he would address the subject of friends and allies.

A draft of the Chicago speech was prepared by one of Reagan's well-balanced foreign policy advisers, Richard Allen, and sent to Santa Barbara, where the governor was vacationing. Reagan found the text dull and began fiddling with it himself. He turned for help to some of his ideological advisers. As a result, the language was made more spicy, and the thrust changed.

Peace, to be sure, was the central theme. "Clearly," Reagan said, "world peace must be our No. 1 priority. It is the first task of statecraft to preserve peace."

In addition, Reagan made a good rebuttal to the charge his election would promote an arms race between the United States and Russia. "We are already in an arms race," he said. "But only the Soviets are racing."

But then Reagan introduced a wholly gratuitous reference to Vietnam. He called the war "a noble cause." He strongly implied that victory would have been possible if only "our government" had not been "afraid . . . to win."

Thus, instead of uniting the country behind a prudent policy, he worked to redivide the country on a subject famous for fostering the most furious discord. At the same time, by implying the war had been winnable, he reopened an endless debate that leads nowhere.

The diversion is the more grievous because there are truly important defense issues at stake. The fact is the United States is not well equipped to meet its security responsibilities. Nobody knows it better than the Carter administration -- which is why the president and his assistant for national security affairs, Zbigniew Brzezinski, have laid down a veritable artillery barrage of bogus claims and cloudy stories.

For example, there is the president's ceaseless talk of a rapid deployment force -- which, far from being a force, is merely a headquarters. There is the leak about a new bomber -- called the Stealth because it is designed to minimize detection by radar -- whose future development has apparently been made known in order to justify the administration's cancellation of the B1 bomber.

Then there is Presidential Directive 59, which according to some versions, enunciates a new strategic doctrine that stresses nuclear bombardment of Russian military targets, as distinct from large cities. The only reason for putting the story out at this time is to give the impression that something big is happening in the defense field, when, in fact, almost nothing is occurring.

But the genuine case against the administration cannot now be effectively pressed home. President Carter can plausibly argue that Reagan only raises the security issue for partisan political purposes. The Democrats can fairly claim that Reagan is a beaky hawk who cannot be trusted to have a finger on the nuclear button.

Nothing in this is particulary surprising. However solid his instincts, Reagan is over his head in dealing with much of the serious business of government, including national security. It figured all along that he would make the kind of blooper he made in the Chicago speech.

But no one should be deceived into thinking that Reagan's wrongs make Carter's policies right. In fact, the Carter administration has presided over a diminution of American strength. It has played politics with national defense -- to the point where serious men could believe the story that Carter would order an invasion of Iran in order to help his election prospects. All of us, accordingly, suffer from the thoughtless way the defense debate has been broached by Reagan. The incident is one more bit of evidence for the proposition that while Carter may not be able to win, Reagan can certainly lose.