The American Bar Association added its voice to the nuclear arms debate today by calling on the United States and other countries to pursue "serious and sustained negotiation" and to prevent the further spread of nuclear weapons.

The ABA House of Delegates, meeting here for the bar's annual convention, also voted early this evening to put off until February a decision on a controversial set of lawyer ethics rules. That vote came after the group spent nearly four hours debating some of the rules, which have stirred a bitter fight among various factions.

The cautiously worded nuclear arms resolution stopped short of endorsing a freeze or unilateral reduction. Nonetheless, the measure placed the 280,000-member ABA on record for the first time in support of steps to contain the spread of nuclear weapons.

One provision has been interpreted as a slap at the Reagan administration because it calls on countries to "avoid conduct and rhetoric that invite nuclear confrontation and obscure their mutual interests in reducing the risk of nuclear war."

Some lawyers have warned that statements by some administration officials have done little to reduce the risk of nuclear war.

Although the resolution passed easily on a voice vote, critics said the issue should have been left to defense experts rather than lawyers. Joe Stamper, of Antler, Okla., said the measure called on the United States to "unilaterally disarm."

Coming after only a short debate, the ABA's vote marked a victory for the Lawyers Alliance for Nuclear Arms Control, a Boston-based group seeking a freeze on nuclear weapons.

"The ABA's position is tremendously significant because it brings into the mainstream of the legal profession an issue which for the last 37 years has been outside the minds of most people, including lawyers," said Alan Sherr, a Boston lawyer and president of the alliance.

In another action, the ABA body retreated a bit from a position it took last year that allowed law schools to discriminate on the basis of religion. While the ABA voted to continue that practice, it also required schools to have a "diverse" student body in terms of race, sex and religion.

The religious issue has embroiled the ABA ever since Oral Roberts University, a fundamentalist Christian school in Tulsa, sued the ABA to prevent it from denying bar approval for its law school, which requires prospective students and faculty to sign a pledge recognizing Jesus Christ as the "Lord and Savior." Prior to the suit, the ABA did not allow law schools to have any sort of religious preference for faculty and students.

By amending its rule, the ABA made it clear that schools could include religious preferences "only to the extent that they are protected by the United States Constitution."

The rule change could jeopardize the Oral Roberts school's ABA approval.

Earlier today, U.S. Attorney General William French Smith told the ABA that the administration was continuing to press Congress for changes in federal criminal and civil laws, which he criticized as being "tilted too decidedly in favor of the rights of criminals."