President Reagan's contemplated tuition tax credit for private schools came under instant attack yesterday, even before its formal unveiling in a scheduled speech tomorrow.

Administration officials said that, under the current plan, families would be allowed a credit equal to 50 percent of tuition paid up to a maximum $100 credit in fiscal 1983, $300 in fiscal 1984 and $500 thereafter. Families with incomes up to $50,000 a year would be eligible for the full credit, and those up to $70,000 for some part of it as now envisioned.

Congressional leaders who have supported tuition tax credits in the past, such as Sen. Robert J. Dole (R-Kan.), chairman of the Senate Finance Committee, and Rep. Barber B. Conable Jr. (R-N.Y.), ranking minority member of the House Ways and Means Committee, both were said by aides yesterday to oppose such credits now because they would fly in the face of rising budget deficits.

And even some who do not share this objection feel it is too late for this year. Sen. Daniel Patrick Moynihan (D-N.Y.), co-sponsor of a tuition tax credit bill, said yesterday that the president's action "is greatly to be commended. There is, however, some ground for concern. The administration has waited so long to make its intentions known, it may prove to be impossible for this Congress to deal with this issue in the time remaining."

The tuition tax credit idea also is sure to draw fire as it always has in the past from public school people who have seen the Reagan administration propose large cuts in federal aid to their programs, and from those who see the credits as violating the Constitution's separation of church and state.

But the tax credit proposal, as administration officials pointed out yesterday, will fulfill the president's campaign promise to back the idea, and, it is hoped, will shore up his support among Catholic voters.

Frank Monahan, of the U.S. Catholic Conference, said yesterday in a speech in Chicago that the president will have to lobby hard for his proposal once he makes it. "Unless this is done, the Reagan tax credit bill will be perceived on Capitol Hill as an administration going through the motions to give the appearance of making good on a campaign commitment."

"In politics, perceptions are as important as realities and if the administration's support is seen as weak it will be dealt with accordingly . . . .Our task is to keep everyone's feet to the fire, the White House, the Congress, Democrats and Republicans alike."

The details of the proposal the president is expected to outline in a speech to Catholic educators in Chicago still are not final, administration officials said. They said it will be phased in to reduce the revenue loss to the Treasury; will not apply to colleges, as the Moynihan bill does; and will contain a section making it clear that the credits will not go to segregated private schools.

Russell Shaw, secretary for public affairs at the U.S. Catholic Conference, said his group viewed the president's expected proposal as encouraging, but added: "We're not too keen on the timing. We'd rather have seen it in January."

Shaw said he didn't believe enactment would result in an explosion of enrollment at Catholic schools. "It has more to do with their being able to hold their own," he said. There are about 5 million students in private schools. About 3.1 million were in Catholic schools in 1981, compared to about 4.4 million a decade earlier, he said.