President Carter's plan to increase federal scholarships and loans for college students apparently hasn't dampened pressure for the rival tuition tax credit - at least among the lawmakers charged with writing tax legislation.

In an opening day of hearings on the tax credit proposal, members of the House Ways and Means Committee indicated they still generally favor the tax credit plan and are skeptical of enlarging existing scholarship and loan programs.

At the same time, the tax credit proposal won the backing of the influential Catholic school lobby, which favors a version that would aid parachial school pupils. Two key Catholic school teachers' organizations joined the U.S. Catholic Conference in supporing the plan.

Yesterday's reactions do not mean Carter has lost his bid to head off the tax credit proposal. House and Senate education committees are expected to move quickly to push through the scholarship increase program before the tax credit plan comes to a vote.

Rep. Al Ullman (D. Ore) chairman of the Ways and Means panel, is expected to put off formal action on the tuition credit to give the education committees time to push through Carter's alternative. However, conservatives may try to add the tax credit to some other bill.

Joseph A. Califano Jr., the secretary of health, education and welfare, reminded the committee yesterday that Carter has insisted that the nation "can't afford" both the tax credit and increased scholarship aid. However, he declined to say whether Carter would veto a tax credit bill.

Califano also estimated that if Congress enacted a broadened tuition credit that covered elementary and secondary schools as well as colleges, it would give families of students attending private schools more federal aid per person than those attending public schools.

The cool reception given Carter's scholarship aid proposal in the committee appeared to cross party lines. Apart from Ullman, the only members to speak in favor of increased scholarships and loans were Reps. Charles B. Rangel (D. N.Y.) and Fortney H. (Pete) Stark (D. Calif.).

The two Catholic school groups adding their support to the tax credit proposal were the National Association of Catholic School teachers and the Federation of Catholic teachers. Earlier, the U.S. Catholic Conference and several other groups endorsed the bill.

Carter's proposal would increase existing aid for scholarships and student loans by $1.46 billion - largely to channel more money to middle-income students, but only $215 million would be spent in fiscal 1979, however. The bulk of the program would take effect in fiscal 1980.