The national campaign to "freeze" Soviet and American nuclear arsenals has been formally endorsed by nearly half the country's Roman Catholic bishops, the auxiliary bishop of Newark, announced yesterday.

The Rev. Joseph A. Francis of Newark, representing the Catholic peace group Pax Christi, told reporters that 133 of the country's 280 active bishops have endorsed the bilateral freeze.

Two months ago, just 69 Catholic bishops had endorsed it. Francis predicted that nearly every bishop in the country will be supporting the freeze by November, the time of the next national bishops meeting.

Francis spoke at a press conference called by the national freeze campaign to draw attention to its latest endorsements and to challenge President Reagan's contention that the United States is now inferior to the Soviet Union in strategic nuclear power.

Randall Forsberg, a founder of the freeze campaign, said 69 city councils had endorsed the freeze, as had with 22 county councils and eight state legislatures.

Rep. Edward J. Markey (D-Mass.), sponsor of a pro-freeze resolution in the House, said he had 168 cosponsors, and he predicted the resolution would pass the House this session.

Markey said the support of Catholic bishops and others in the church is one sign that the freeze movement is a mainstream phenomenon sweeping the country.

Reagan opposes the freeze because he says it would lock America in a position of inferiority. Forsberg, an arms-control specialist, said this view is wrong.

"Both sides' nuclear forces are partially vulnerable," she said, but neither superpower has the capacity to launch a "disarming first strike against the other."

Also yesterday, 150 students and 20 professors from the Harvard Medical School lobbied members of the House and Senate and administration officials on "the disastrous medical consequences of nuclear war."

Most injured persons in such a war "would never see a doctor," said Daniel Lowenstein, a third-year Harvard medical student.