It is too late to save the president's resigning national security adviser, Robert C. McFarlane, from what befell him at the White House. But there may still be time to save him from the fate that is getting ready to befall him now: transformation by administration critics on the left into someone he never was -- a kind of doomed and valiant closet dove shoved out by the forces of reaction and darkness. By about Friday, we should guess, the aforementioned dark forces will of course be striking back, countering that Mr. McFarlane, mourned by so many of the administration's ideological foes, can hardly have been the right man for the job. Et cetera.

Let us try a little pre-emptive strike here. Mr. McFarlane is no dove -- doomed, valiant, closet or otherwise. He is a very conservative and sober- minded military officer turned civilian, who has become a specialist in national security policy. The job he had has always been a delicate one to fill and has regularly been redefined by those who held it. Some were more and some were less assertive, intrusive, imaginative, self-starting and egomaniacal. McGeorge Bundy, Walt Rostow, Henry Kissinger, Brent Scowcroft, Zbigniew Brzezinski, Richard Allen, William Clark -- you don't exactly find a pattern there.

Some of these men were more illustrious than Mr. McFarlane, but we can't think of any who was more helpful to the president he served. This, we suspect, will be noted as time goes on. By the time Mr. McFarlane took the White House job, a major task awaited: to find a way to turn the fruits of Mr. Reagan's military buildup into actual policy options. He gave over a great deal of time and thought to this. Mr. McFarlane also had some successes as a manager and arbiter of departmental clashes. He has been the kind of publc servant who is not fully appreciated until he is no longer around to do the countless quiet, essential tasks that an employer tends to take for granted. He has been that rarest of public officials, a loyal, honorable and unassuming man who was also intelligent and tough.

A near-frantic effort has been mounted by persons close to White House chief of staff Donald Regan to counter the stories that Mr. Regan's manipulating and muscling had a part in Mr. McFarlane's decision to leave. And yesterday both Mr. McFarlane and the president dismissed the reports as nonsense. We wish they had been nonsense. The muscling and manipulating were particularly egregious, and they did have an effect.

Neither Mr. McFarlane's job nor that of Mr. Regan is subject to Senate confirmation, and a president has the widest possible discretion in choosing the persons he wants for them. Chemistry, as it is called, work habits, style and personal quirk all play a part, and for all we know Mr. Regan may suit Mr. Reagan's needs just fine. But from the outside it sure doesn't look that way. His ascendancy has been one grating episode after another -- George Bush, Margaret Heckler, Robert McFarlane -- just as his Cabinet years were marked by open combat with others high up in the Reagan government. Mr. Regan is a very ambitious man. He is said to like to be seen in the right places at the right time. The stories are legion. At Geneva he had himself photographed draped over the back of the couch on which Ronald Reagan and Mikhail Gorbachev were sitting. Mr. McFarlane is just the opposite, and you would think that the self-effacing man would have presented no particular challenge or threat to the other. But it seems not to have been regarded by Mr. Regan that way.

Mr. McFarlane's successor is to be his deputy, Vice Adm. John Poindexter. Good luck. That the choice evidently had to be agreeable to Mr. Regan as well as to the president puts a couple of extra bricks in the new fellow's knapsack. But Adm. Poindexter must know, from his time in the White House, how great is the president's need for someone capable of performing the crucial balance- wheel function defined and assumed by Mr. McFarlane.

At the summit, the president put himself in a way to move toward major policy decisions. But he has not yet made those decisions. In the absence of a McFarlane, the internal chemistry of the Reagan administration's policy process will be different. Let us hope it works.