The Senate Finance Committee, openly defying President Carter, approved legislation yesterday that would give a tax break this year to parents of college students and would extend it in 1980 to cover elementary and secondary school pupils as well.

The double-barreled measure effectively combines two major tuition credit proposals -- a college credit proposed by Sen. William V. Roth Jr. (R-Del.) and a broader version by Sens. Bob Packwood (R-Ore.) and Daniel Patrick Moynihan (D-N.Y.).

The surprise joining of forces, worked out Wednesday night, is expected to deal a serious setback on efforts by Carter to head off enactment of a tuition tax credit, which the administration considers too costly and wasteful.[PARAGRAPHS ILLEGIBLE]

However, the measure was endorsed by the Catholic school lobby, which is seeking relief from rising tuition burdens. The Rev. Patrick Farrell, spokesman for the U.S. Catholic Conference, said the bill provides "hope that a realistic choice in education will be maintained."

The breaks would come in two stages: Starting next Aug. 1, the bill would offer a tax credit of up to $250 a student for tuition costs of college undergraduate and vocational-school students.

Beginning two years later, the credit would be raised to a maximum of $500 a student, and the tax break would be expanded to cover elementary and secondary school pupils -- including those in parochial schools --and college graduate students and part-timers.

The Finance Committee approved the bill yesterday by an overwhelming 14-to-1 vote. The panel agreed to attach the credit to a minor wool import duty bill, which is scheduled to come to the floor soon. Passage by the Senate seems virtually assured.

The only senator on the panel to oppose the tuition credit was Sen. Lloyd M. Bentsen (D-Tex.), who expressed fears it might prove inflationary. Sen. Harry F. Byrd Jr. (Ind.-Va.) voiced similar concerns, but voted "present" --

Yesterday's action now pits the tuition credit in a race against Carter's own alternative proposal -- a bill to expand existing federal scholarship and loan programs to benefit more middle-income students. The president has opposed aid to parochial schools.

Key House and Senate Education committees currently are working to rush the Carter package to the floor. The president has warned he will "not accept" both the increased scholarship aid and the tax credit, and has implied he will veto one or the other.

Along with the tax-credit provisions, yesterday's legislation contains several proposals designed to deal with two of the most serious concerns about the tuition credit -- the question of aid to parochial schools and the failure to provide benefits for low-income persons.

The committee included a provision that would invite an early court test of the constitutionality of tax credits for tuition paid to parochial schools --with a view toward getting a ruling on the question before that portion of the aid goes into effect, in 1980.

It also agreed to provide cash payments to families too poor to pay taxes and which would not ordinarily qualify for a tax credit. Congressional staffers said, however, that the aid under this category would be relatively modest.

The committee also agreed tentatively to ask the Budget Committee to allow room in the fiscal 1979 budget resolution for a net tax cut of $25 billion -- about the amount proposed by the Carter administration.

However, its action yesterday does not mean the panel will go along with the specifics of the Carter tax package. Indeed, committee members hinted yesterday they may use $1 billion of the total to finance the tuition credit for fiscal 1979.

The $250 and $500 a student limits on the tuition credit refer only to the maximum tax break allowed. Specifically, the credit for 1978 and 1979 would amount to 50 percent of the first $500 in costs. For August 1980 and later it would be 50 percent of the first $1,000.

The committee made public new estimates yesterday that showed all but 4 percent of the benefits from the tax break would go to families with annual incomes of $50,000 or less. The bulk would be concentrated in the $15,000-to-$30,000 range.