Fifty-three pages into Sen. Gary Hart's 54-page foreign policy pronouncement, the thing gets really interesting.

I'm speaking of the tome Hart submitted this week (in the form of three lectures) as evidence of his worthiness to be the next Democratic presidential nominee. The Colorado Democrat is a sober and brainy fellowju and leader of the Democratic pack; his views have got to command duti-ju ful respect. Up to page 53, however, the emphasis is on the dutiful. Hartju is full of intelligent formulationsju and humane sentiments. But there is not much evident zest for whatever he happens to be discussing at the time.

Then, almost at the end, something happens. It's as though Hart changed voices, or speech writers. Suddenly, right after the part that says "Americans will welcome the waves of change and steer confidently ahead," he launches into vigorous rebuttal of the charge of isolationism.

It's a sharp reminder that, in the second decade after Vietnam, Democrats attempting to lead their party to victory wisely concede a requirement to deny that their policy signals an international retreat. Hart, who was chairman of the 1972 McGovern campaign, in which the candidate pleaded, "Come home, America," has a special problem. But almost the whole party shares it. Through the 1970s Democrats felt that the principal danger was "involvement" on the Vietnam model. That remains one of the party's principal fears now in respect to Nicaragua.

In the 1970s, the Soviets took advantage of American distraction and made substantial strategic and geopolitical gains. By so doing, they lit the political fires that brought Ronald Reagan to the White House. That left the Democratic Party with a continuing need to be tough and plausible on the Soviet issue, in a way that does not exactly mimic the Republicans, and at all events to avoid leaving itself open to the charge that it's still withdrawing from the world.

In truth, Hart, in the first 53 pages, steepens his own path. His big theme is that superpowers no longer dominate the world and "the diffusion of power is the defining reality of our age." This is one of those amorphous generalities that seem to come with the Democratic Party card. In that way Democrats mean to set themselves off from Republicans, who are said to be too Moscow-minded.

In fact, it might be a nicer world to live in if the Democrats were right and superpower relations were not at the center of things. That would mean that those relations had improved and it was now not only necessary but safe to give priority to the vexations of diffused power. But I doubt that's the world we live in. Much power is diffused, but Moscow is the main threat. In the particu-ju lars, Gary Hart understands this well. In the large, however, he bows toju a Democratic sensibility and mumbles earnestly about the diffusion of power.

But finally he comes to the gut fear that Democrats will be accused of favoring "retreat, withdrawal, isolationism." Those who say so, he declares, couldn't be more wrong. He then boldly attempts to pin the iso-ju lationist label on his would-be accusers.

The real threat of isolationism, he states, comes from those who would "elevate their rejection of arms control into an ideology that destroys prospects for a more stable world," from protectionists, "from those who would neglect diplomacy and thus frighten our people into believing that internationalism will always entail loss of American life," and so on. He terms his own brand of internationalism "enlightened engagement."

This is not to say that all of Hart's policy prescriptions are on the mark. Surely he is right, however, to be out there looking for terrain where a post-Vietnam liberal internationalism can be built.

Ronald Reagan is not the isolationist that Hart's rhetoric implicitly makes him out to be. But the president's declared readiness to commit American power -- a proclivity applied with more restraint than many had expected -- has an unmistakable go-it-alone edge. This is the contemporary form of conservative isolationism.

The more familiar liberal isolationism is what Hart has to shake -- in word, in political imagery and the nature of his following, and in deed. It is not something that can be completed even in a 54-page speech delivered over three days, but he is right to be setting about the task.