President Reagan is to a great extent a victim of his own success in defense. The buildup of the last five years is one of the distinguishing achievements of his administration, part of his signature in history. But the rising defense budgets that are the measure of the buildup have helped reduce the very apprehensions that produced them. At the same time they have brought problems of their own. The president's speech in behalf of his new defense budget Wednesday night was an effort to offset both these realities. But a speech can't do it.

The president was right in saying that, ultimately, the defense budget should be driven by the threats it is meant to combat. But the very accomplishments he boasted of -- the restoration of national self-confidence that in some ways is his premier accomplishment -- have made the threat appear to recede. "Tonight," he said, "the security program that you and I launched to restore America's strength is in jeopardy -- threatened by those who would quit before the job is done." But to a large extent the people to and about whom he spoke were the same.

The speech was an effort to change the focus of debate, but it went too far. The rock the defense buildup has hit is the deficit, which it also helped create. The president did not mention this, said not a word about the trade-off of social programs for defense that he continues to propose, nor explained why the need to continue the buildup is not matched by a need for a tax increase. He was dismissive of the problems with the procurement proce objections of his own valued defense secretary, to appoint the special advisory commission that is to report to him today. Cost growth is down, he said, most of the horror stories of recent years have been the result of the Pentagon's own renewed vigilance, and if further reforms are needed, he will impose them. The president missed what is in people's minds, and that is unusual for him.

On the deeper questions -- how well do the weapons work, are they the right ones in the right combinations, and if they are not all affordable, which can the services do without -- he was merely enthusiastic. For example: "today's technology makes it possible to destroy a tank column up to 120 miles away without using atomic weapons," he said, thus providing perhaps "the first cost-effective conventional defense in post-war history against the giant Red Army." Failure to spend on such weapons, he continued, only increases the risk of nuclear war.

The president suggests in such as this that only two choices exist -- the budget as he presents it or a giving of ground to the Soviet Union that risks negating all the gains of the last several years -- gains that we agree are real. But that is a false choice. There is a middle ground that both Congress and the country -- all of us -- are now reaching for. The president should be helping to find it. His speech the other night blocked the way.