The Reagan administration is now playing catch-up on arms control. Fortunately, it's got a powerful prospective ally who could help it come out better than it might ever have done on its own.

The administration originally thought it had a lot of time to put together a full arms control position. So it dawdled, rationalizing that meanwhile it was putting money in the bank by raising the defense budget and talking tough, and it spared itself the rigors of resolving its internal arguments.

When, predictably, the Europeans panicked at this display of nonchalance, President Reagan produced a part of a position, on medium-range missiles, but otherwise kept dawdling.

Not long ago I asked arms control chief Eugene Rostow how things were going and he said the experts were still working to define a new "unit of account"--a way to measure strategic forces to replace the old now-spurned way of counting launchers and missiles. Six or eight approaches were being studied, he said; the interagency process was coming to a head.

Well, large swaths of the public and Congress got tired of waiting. Doubt in the administration's competence and good faith is now flowing freely. The support for a nuclear freeze shows it, and so does the resistance to the president's defense budget, and to such line items as his MX proposal.

A year ago the president could have framed arms control proposals reflecting both his electoral mandate for a hard line and the broad popular acceptance he then enjoyed. Now his national security mandate is clouded by misgivings about his commitment to keeping the nuclear peace, and his popular acceptance has faded.

In these circumstances, to frame the sort of arms control proposals he might have made a year ago, while the public was more of a mind to back a hang-tough president, is to invite a damaging argument in this country even before the proposals reach the Kremlin. That means the administration is going to need all the help it can get to win the degree of popular support that will make the Soviets take seriously its proposals for START-- the Reagan replacement for SALT.

Enter Edmund Muskie. At 67, the former Maine senator and last Carter secretary of state, now practicing law, has the years of engagement in the issue, the savvy and the stature to be a key figure in the gathering arms control debate. He and his moderate kind are well situated to help deliver to Reagan what the president will need in full measure in the months ahead: a windbreak on his left to check the impatience, anxiety and outright fear that an increasing number of Americans feel toward Reagan's nuclear policies.

Muskie's special gift is to combine his expertise and political sense with a deep sense of the trust held by the few people who are in a position to push the button. A man beyond posturing, he is respectful not simply of national interests but of universal human interests. He has precisely that extra dimension that so many people sense is lacking in Reagan and some of his ostensibly hard-headed advisers, and it came out in a marvelous speech he delivered last week in Washington, and in a subsequent interview.

Muskie seeks a "diplomatic 'middle way' ": neither arming to the teeth nor falling into unilateral disarmament. He accepts the need to maintain a nuclear arsenal, and the need to conduct a "credible search for answers to the dangers of uncontrollable competition." He thinks the recent freeze proposal raises questions as a policy but could rebuild the requisite political base for arms control for a president willing to embrace a sensible, negotiable program.

Muskie is not one of those liberals who have already excommunicated Reagan on arms control and gone to the trenches. He takes the view that, despite some strong contrary indications, the president has not yet lost the chance to get on the right track. He offers him in effect his support in staving off popular emotional pressures on the left, if Reagan will adopt the moderate program still within his reach.

The danger for the president, Muskie feels, is that Reagan will find himself backed into a corner, offering, finally, proposals that satisfy one part of his constitutency but take the country only to deadlock and danger. Muskie and others like him could help Reagan escape from that corner and enable him actually to pursue the arms control objectives he professed in his November speech. It depends completely, Muskie told me, on Reagan.