President Carter needs to settle an urgent $300 million tax problem that has already hit more than a million Americans, many of them sick, disabled or retired on small pensions.

The people involved - thands to a mistake by Congress - have been socked with unexpected 1976 tax bills ranging from $50 to $1,900 each. The bills are due by midnight Friday.

The problem arose when Congress, late in 1976, voted to wipe out - for the entire year of 1976 - a heretofore legal, $100 a week sick or disability pay tax break. Almost as soon as the tax reform act was passed, Congress began to try to clarify its mistake. But it left town last week with the clarifying legislation - called the Dole-Daniel bill - unresolved.

Although both the Senate and House approved the Dole-Daniel bill without a single dissenting vote, the Senate balked at giving the House version final approval on April 7 after Finance Committee Chairman Russell Long (D-La.) said President Carter might veto it.

(The changes involved language unrelated to the tax-clarification matter.)

Long says that the Treasury Department opposes the bill on grounds it would cost something over $300 million in tax revenue this year. Judging by the ill will that the precedent it sets for all of us in future tax "reform" action - the Carter administration would be wise to forget the $300 million. Since Congress made the original mistake, maybe it could come up with a new source of revenue.

Because Congress left down without taking action on the corrective Dole-Daniel bill those 1.5 million people involved must go ahead and pay their 1976 taxes on time, and pay the extra money they owe because Congress changed the rules on them.

They will be able to have it refunded - if the law is changed - but they must pay out in the meantime. Incredibly, some people will probably be hit with interest penalties form the government because those who pay quarterly estimated tax bills underestimated their tax bite because Congress made a mistake.

Very few people argue that Congress did wrong when it wiped out the $100 a week federal tax exclusion on sick and disability pay for most people.

But nearly everybody agrees that Congress did the wrong thing, at the wrong time, by making the changes retroactive. As it is, people will have to pay extra money, and penalties in some cases, even if they do get some kind of refund later on.

Some people are talking about a lawsuit to test the constitutionality of Congress making retroactive changes in the law. It would be an interesting case. Also an expensive one.

An easier way would be for the President to back the Dole-Daniel bill, and put IRS, Congress and taxpayers on notice that he wants it passed and sign it. In a letter to the President (dated April 8) Dole and Danie said:

"It is our sincere hope that you will overrule the objections of the IRS and affirm your own support of this needed legislation . . . Your commitment would allow thousands of hardpressed taxpayers to use the filing deadline extension already available and to pay a lower estimated tax - confident that the Congress and President will soon enact a change in this retroactive, unnecessary, and harshly unfair tax burden . . . " The deadline is April 15. We'll let you know as soon as Dole or Daniel gets a letter from the White House.

Joseph D. Gebhardt, public interest lawyer in various citizen-bites-government legal cases, says he will run next year for the Democratic nomination for the Maryland State Senate.