BAD IN PRINCIPLE from the beginning, the tuition tax credit bill is now about to come to the Senate floor by an appropriately dubious route. The single thing to be said for last week's maneuvers is that they have drawn attention to the bill's defects. Those defects are numerous, profound and far beyond the remedy of any routine amendment or fiddling with wording.

Sen. Ernest F. Hollings (D-S.C.) puts the issue accurately in the article that we print today on the opposite page. The bill proposes giving federal aid to anybody who pays tuition, with the amount of aid based not on families' need but on the size of the tuition. That's unacceptable, but, as far as college students go, it's a secondary matter. The central danger in this bill is the unrestricted federal aid that it would extend, for the first time, to every kind of private elementary and secondary school.

This legislation would let people deduct part of any tuition payment from their federal income taxes. The federal government would, in this fashion, pay part of every tuition bill. The device is known as a tax subsidy. The Senate Finance Committee, that haven for every kind of special pleading, reported a bill last winter but never took it to the floor. It was waiting for the House, which passed a similar bill in June. Under that version, the U.S. Treasury would by 1980 pay, through tax credits, one fourth of every student's tuition up to $250 in colleges and $100 for schools. The cost would be $1.1 billion a year.

What about people with incomes so meager that they pay little or no tax, and have no use for a credit?The House, to meet that objection, wrote in a refund clause giving cash to anyone whose tax is smaller than the credit. The cost is small; the importance of the refund was that, when the bill got to the Senate, it was required to go to the Appropriations and Budget committees as well as to its warm friends in the Finance Committee.

On Tuesday, the Appropriations Committee voted a negative report - a recommendation to the Senate to kill the bill. On Wednesday, the Budget Committee voted a negative report, citing not only the bill's cost but also its radical and ill-considered reversal of longstanding American values. On Thursday, the Finance Committee hastily reported out a sustitute bill, dropping the refund clause to avoid the other committees. That is the bill that will come to the floor - perhaps today. The new bill evades the other committes' jurisdiction, but it does nothing to meet their objections to it.

Since President Carter has said that he will veto this bill, some senators have apparently concluded that it offers a free ride - an opportunity to please some of their constituents, while relying on Mr. Carter to save them from the consequences. That's too easy. This bill is a fundamental assault on the public schools. Any vote for it in the Senate is a threat to the public school system. Regardless of the bill's outcome, the people who support it can properly be held accountable for their votes in precisely those terms.