THE STRUGGLE IN CONGRESS over tuition tax credits is turning into the most complicated kind of mess. But if you inspect it carefully, you will find that it reflects several highly interesting patterns in current American politics. There is the new congressional anxiety to placate the middle-class voters, who are angry about taxes. There is the rebellion against official regulations, and the endless filling out of long forms, that the government has to require for the traditional federal grants based on the applicants' need. The appeal of the tuition tax credit is that it just throws a little money at everybody who pays tuition, whether they need it or not, and leaves it at that. There is also the Carter administration's response, as usual correct in principle but unconvincing in execution. To judge the scale of this question, keep it in mind that more than half of every year's high school graduates now go on to college; college and university enrollments are now well over 10 million.

In January the tuition tax credit bills were off like a shot and several senators, perceiving this momentum, tacked on an amendment expanding it to cover private and paraochial elementary and high schools. That brought in all the familiar religious and racial issues. Meanwhile, the Carter administration responded with a counterproposal to expand the present system of federal grants and loans to college students, extending them for the first time to youngsters from families well up in the middle-income range. The administration bill has been reported in both the House and Senate, but in both places it has been shunted aside until the question of the tax credit has been settled. The administration's preemptive strike has, in effect, been preempted. Unfortunately the administration has further weakened its position by neglecting to send Congress a formal budget request for the grants, raising doubts about the firmness of its intentions.

The tuition tax credit could come to a vote in the Senate as early as Monday, when the first budget resolution for fiscal 1979 is to come to the floor. The resolution provides half a billion dollars in revenue losses for the first year of the college tuition credit; by 1980, the Senate bill could cost perhaps 10 times that much. In the House, a more restrictive bill is now before the Rules Committee.

Colleges and universities have denounced the tuition credit as thoroughly bad in principle - and they are right. Some of the reasons are elaborated in the letter from five college presidents that we publish on this page today. But the credit idea is getting a lot of support from people throughout the country. There are a couple of painful points that, we suggest, they have not considered sufficiently.

Over the past decade, colleges have been pretty careful about raising tuitions and other costs no faster than the inflation rate. But a $500 tax credit would mean that they all could raise tuition at one sudden bite without actually increasing the strain on their students' families. That credit would be an almost irresistible invitation to the colleges. The families getting the tax break wouldn't keep it for long.

But if this multi-billion-dollar tax break gets enacted, who do you suppose will pay for it? Congress is certainly in no mood to increase the budget deficit to accomodate it. Instead, Congress is much more likely to compensate for this revenue loss by scaling back the income tax cuts that President Carter is requesting. The Carter tax cut is already in serious trouble in the House. There isn't going to be any net reduction in taxes collected. To give a break to families with children in college, everybody's tax rates would be left a little higher than they otherwise would have been. This truth would, of course, lie deep in the mathematics of all those tables and schedules of the tax forms, where the voters would probably not see it - until, once again, too late.