The Reagan administration sent two Cabinet members to Capitol Hill yesterday in a show of support for its proposal to give tuition tax credits to parents with children in private schools.

Still, other advocates of the measure were unsure whether the backing was anything more than symbolic.

Treasury Secretary Donald T. Regan and Education Secretary Terrel H. Bell told the Senate Finance Committee that the president would lobby hard this year for the bill.

The credit would start at $100 per child in 1983 and increase to a maximum of $500 or one-half of tuition costs by 1985. It is expected to cost $1.3 billion by 1987.

But Regan said the bill should not be attached to the committee's bill to raise taxes by more than $20 billion. The tax increase to offset rising federal deficits is the committee's chief preoccupation now.

Lobbyists for the Moral Majority and the U.S. Catholic Conference said privately that they had no commitment that the tax credit bill would be pushed this year.

Sen. Robert H. Dole (R-Kan.), chairman of the committee, said he supported the concept of tuition tax credits, but noted that "immediate action on any new or expanded tax expenditure with significant revenue impact may not be possible."

Ronald S. Godwin, vice president of the Moral Majority, said he didn't want to commit his group to a full-fledged lobbying effort for the bill until he was convinced the administration is serious. "Otherwise this is just window dressing," he said of the hearing, which drew a standing-room-only crowd and a flock of reporters and television cameras.

Sen. Bob Packwood (R-Ore.), who has pressed similar legislation unsuccessfully for five years, told Regan that to pass the bill the administration would have to lobby as hard as it did for selling Airborne Warning and Control System (AWACS) equipment to Saudi Arabia. "The administration has to want it desperately," he said.

A parade of other witnesses repeated the arguments for and against the concept. Supporters said the bill would give parents a choice of where their children should be educated without harming the public schools. Opponents attacked the bill's cost and constitutionality.

Committee members indicated that the administration bill probably would be amended. They questioned the strength of provisions to bar schools that discriminate from benefiting from the credits. And some expressed doubt about the decision to let families making up to $75,000 a year get part of the credit, while those too poor to pay taxes are excluded.

Sen. Bill Bradley (D-N.J.), who supports the concept of tuition tax credits, asked Regan why the administration proposal allows the full credit for families making $50,000 a year. Regan first called the $50,000 a "median income."

But when Bradley shot back "For what group?" Regan said the $50,000 was an "average income," and then said, "You have to go somewhere. That was picked as an appropriate figure."

Regan also appeared to have difficulty answering Bradley's question about how the bill's discrimination provisions would be policed. He said discrimination statements would be examined by Internal Revenue Service agents, then said the government would rely on complaints from parents.

Many church groups have opposed IRS efforts to police discrimination regulations in tax exempt private schools as government interference.

Bell said at one point in his testimony that the federal government now spends about $330 per student in the public schools. Sen. Ernest F. Hollings (D-S.C.), who led the battle against the credits in 1978 and testified in opposition, said federal aid to education amounts to only $128 per public school student.

Hollings scoffed at the idea of considering reduction of revenues through tax credits when the committee is trying to raise revenues with tax increases. He labeled the entire administration effort in pushing the bill "outrageous and blasphemous" at a time when it was trying to cut other aid to education.

The Most Rev. James P. Lyke, representing the Catholic bishops of the United States, said his group would like to see the bill amended to allow credits for college students as well. He also said it should include a "refundability" clause to pay parents who don't make enough to qualify for the credit.

Lyke noted that many of the Roman Catholic school students in Cleveland, for example, come from poor families.