Christmas in January?

President Reagan has said before that the economy would be doing better if Congress had given him the full 30 percent income tax cut he wanted when he wanted it. So it is not out of keeping for his administration to float the possibility of speeding the coming midyear's last 10 percent of the 25 percent that he did get.

Would such a Christmas in January rather than July offer Americans enough cheer to boost business during the current holiday season? Or would it dampen enthusiasm by raising the prospect of even higher federal deficits owing to an additional $14 billion loss in estimated revenues?

Certainly this year's tax cut did not appear to stimulate enough economic activity to fulfill the supply-side theory of producing greater revenues even with lower tax rates (offset for many by bracket creep and increased social security taxes anyway). At any rate, on the heels of this year's cut, even Mr. Reagan embraced a new revenue-raising package. And others suggested postponing or even repealing next year's July cut instead of rushing it. Perhaps the White House hopes at least to avoid postponement by talk of an earlier date.

Wise words of caution are reported from Budget Director David Stockman and White House economist Martin Feldstein. They are said to oppose the speedup as worsening the huge deficit and perhaps pushing up interest rates.

Tax-cut sugar plums must not be allowed to divert attention from the fundamental task of bringing down interest rates. As a Morgan Guaranty Trust Company executive said last month, there ''simply is no modern precedent in a recession for the real interest burden that continues to exist.''

Thus Federal Reserve Board Chairman Volcker might do more good than a January tax cut by judiciously pursuing the flexibility for bringing interest rates down that he has recently asserted - in contrast with the Fed's damagingly narrow focus and aggressive onslaught against money growth last winter. Mr. Volcker has also valuably reasserted the need for a continuing battle against the inflation that could undermine emerging economic gains.

To such efforts must be added another thrust more certain than a mere tax cut speedup - the attack on unemployment called for anew by the election returns. The momentum is building for the long-discussed double goal of putting Americans to work at the jobs crying to be done. Repairing the nation's ''physical plant'' including roads and bridges would provide jobs in itself. And the improvements would facilitate business and thus create more permanent jobs.

Much of this could be financed in a pay-as-you-go manner through the modest gasoline tax now being discussed. But it could be hard to sell such a flat tax, even though targeted and called a ''user fee,'' while accelerating an income tax cut favoring upper-income groups.

In short, floating the notion of Christmas in January may be less useful in itself than in stimulating the pursuit of better alternatives. These could give individual Americans the confidence to join in the recovery that is struggling to be born.