It seems a foolish thing to say about an administration that has been around for less than a year and has at least three more to go. But there is a growing sense that both politically and governmentally, the Reagan administration may have entered the most critical six months of its life.

The president's pollster, Richard B. Wirthlin, said just before Thanksgiving that these next six months could be decisive for the political fortunes of the administration and the Republican Party. Wirthlin is worried that if the free-fall recession now under way has not hit bottom and begun to turn around by late spring, then recovery is not likely to be visible enough by autumn to give the Republicans strong ground on which to fight the mid-term election.

The other day, one of the more reflective of the president's senior staff members made a similar point in conversation about the governmental world, arguing that it is in the period from nine to 15 months after inauguration, that "a new president comes to terms with reality."

His comments triggered a faint memory and, sure enough, the files turned up a column by me, published exactly four years ago today on "the second transition" of the Carter administration.

Looking back a year to the 1976 Ford-to-Carter transition, I wrote: "Then, it was a time for announcing 'superb' appointees and 'comprehensive' solutions to long-simmering problems. Now, it is a time for taking a second look at some less-than-superb performers and some comprehensive plans that proved less than compelling in practice."

With the wisdom of hindsight, it is easy to say now that President Carter failed to use his "second transition" opportunity to weed out the weak performers in his White House and Cabinet or to sort out his priority programs from those which he should have learned were unrealistic.

What is striking is that some Reagan advisers --like some of their counterparts in the Carter circle four years ago--recognize this as a critical passage for their president.

Personnel decisions--whether to replace the ineffectual national security and domestic policy advisers, Richard Allen and Martin Anderson, the crippled budget director, David Stockman, or the contentious secretary of state, Alexander M. Haig Jr.--are the most obvious choices facing the president.

But equally compelling are the policy choices involved in the current presidential review of fiscal 1983 budget decisions. For the first time since he became president, Reagan is hearing from men he picked to run Cabinet departments and agencies the argument that they cannot absorb cuts of the scale Stockman is recommending, without crippling programs they believe vital.

The president can respond to their arguments by reiterating his campaign rhetoric about all the "waste, fraud and abuse" in the domestic side of government. Or he can adjust to reality--not by abandoning his goal of budgetary discipline but by tailoring it to the facts to which he is now being exposed in this budget review.

Those who are hoping for the pragmatist in Reagan to prevail argue that the policy problems in the unfolding budget picture are manageable in a four-fold approach:

* Modest reductions in the planned expansion of national defense and the "safety net" entitlement programs like Social Security, both of which the president has rhetorically put off limits.

* A modest increase in taxes, probably through excises, rather than delaying the scheduled individual rate reductions.

* A modest easing of monetary policy, in order to bring down interest rates fast enough to ensure the late-spring economic recovery for which Wirthlin and all GOP candidates pray.

* And a modest shift in presidential rhetoric, to point out that a recession-year deficit of less than $100 billion in an economy of about $3.5 trillion is not by itself a huge inflationary force.

All of these points go somewhat against the grain for Reagan. But the choice, as the pragmatists in his circle of advisers see it, is to make a relatively modest course correction now--or risk letting the policy initiative slip into other hands: to Congress, the interest groups or the political opposition. That is what happened to Carter.

Whether Reagan accepts this advice or follows a different, perhaps more purist or ideological course, one thing is clear: this is, in fact, the most critical time in his presidency.