The nagging question about Edward Kennedy and national security is not whether he can move in from the left toward the center, which obviously he can if he chooses. It is whether the liberal center-left is a reasonable place to be as the 1980s heave into veiw.

Some argue that Jimmy Carter has already provided the answer: he has given the left's defining hopes for detente, world order and American moral leadership about as strenuous a run as one can expect in a society as politically cluttered as ours, and the result in foreign-policy terms has been an expansion of risks and uncertainties and a progressive loss of control. It follows that it makes no sense to send someone else down the same road.

And yet many people are unwilling to concede that post-Vietnam liberal internationalism has been overtaken by the growth of Soviet power, by disorder in the Third World and, for that matter, by the tarnishing of America's moral credentials. These people are variously apologetic and angry about Carter, holding that his personal flaws and misfortunes are something apart from his creed.

Are they right?

It has to be said that many of us, including Kennedy, who have accepted the liberal label are on the defensive. We thought that the Russians, once they joined the big leagues in power, would not press so hard; that the Third World, once its sovereignty was won, would tend dutifully to the uplift of its own people; that Americans would remain a worthy model and patron in the eyes of right-minded people everywhere.

But these expectations have not proven out well, or well enough. Unencumbered by having to explain recent frustrations, conservatives have come on strong. Liberals have had an increasingly hard time making themselves seem relevant to the debate about what constitutes a good defense. Kennedy, if he runs, will have to do better -- not only to unite his own party, which is two parties on security issues, and to rally the broader electorate, but to offer responsible leadership.

The senator is no slouch. Unlike too many liberals, he accepts the legitimacy of defense and the need to cope with it in detail. He is not content simply to offer lazy-minded noes and to moan that others have carried the day.

But his language, his tone, his choice of issues and emphases sometimes suffer from common liberal deficiencies. He says, for instance, that upheaval in places like Iran had "little to do with our own relative strength or weakness" and a lot to do with "internal revolutionary change." Quite so, but this formulation skips over the effect of upheaval on the balance of power, which is what stirs conservatives and weights their alarms.

Similarly, while Kennedy has argued cogently against the M, he dismisses as "frankly ludicrous" the hard-liners' anxieties about a coming threat to American land-based Minuteman missiles. This is, alas, too often the liberal way: to talk issues in position-paper terms and to dismiss too quickly the apprehensions of people on the other side.

This week Kennedy collected his thoughts on the defense-budget increase; he supported the president. He asked all the right specific intellectual questions about costs and missions and the like -- but did not get to the underlying general emotional anxiety about whether there is in fact so strong a flood of Soviet power as to require an extraordinary American response.

I think that liberals, ever optimistic and rationalistic, have to go beyond stating their faith in American resourcefulness and good will. They have to address directly the anxieties of the many Americans who are unnerved, and then show up the weaknesses of the conservative point of view.

These themes are there for the developing: hard-liners, captives of their own jitters, lack an acceptable, positive idea of the kind of world they want to build. They risk missing what opportunities there still are for mutual accommodation and restraint. Their policy prescriptions promise an open-ended if not unending expenditure of resources with only uncertain assurance of gain.

But of course a liberal policy must be more than argument, more than a bright staff and a WATS line to Cambridge, more than hope and a humane outlook, more than a readiness to accept new facts and the need to be strong as well as reasonable. The defining extra element is political leadership -- not to be confused with chest-thumping or show biz or any mere mastery of style. It is here that the liberal option will finally stand or fall.