A divided Supreme Court yesterday nullified a requirement by an international union that restricted candidates for local union office to members who had attended at least half of the monthly meetings for the three years preceding an election.

The court said one effect was to make ineligible for nomination 96.5 per cent of the rank-and-file in an Indiana local of the United Steelworkers of America, AFL-CIO.

A second effect was to force potential-insurgent candidates to plan their candidacies as long as 18 months before an election, "when the reasons for their opposition might not have yet emerged," the court said.

Such effects eliminate the attendance requirement as one of the "reasonable qualifications" for union office permissible under a 1959 law intended to assure "free and democratic" elections, the court held.

A 6-to-3 opinoon written by Justice William J. Brennan Jr. did not say explicitly what a reasonable attendance requirement might be.

For the dissenters, Justice Lewis F. Powell Jr. said that the ruling "could invalidate almost any attendance requirement that served legitimate union purposes."

An AFL-CIO spokesman said that of the estimated 65,000 to 70,000 local of international unions, at least one of four - with a disproportionate share of the nation's 18 million union members - has an attendance requirement.

The ruling was a victory for the Labor Department, which had sued to invalidate the 1970 election of officers in Steelworkers Local 3489. The department contended that the attendance requirement - part of the internaional's constitution - compelled the local to violate the "reasonable qualifications" section of the Landrum-Griffin amendments to the Taft-Hartley Act.

A U.S. District Court judge dismissed the department's complaint but was reversed last year by the Seventh U.S. Circuit Court of Appeals. Voting yesterday to affirm, in addition to Brennan, were Chief Justice Warren E. Burger and Justices Byron R. White, Thurgood Marshall, Harry A. Blackmun and John Paul Stevens.

Joining Powell in dissent were Justices Potter Stewart and William H. Rehnquist. They termed the ruling "an unwarranted interference with the right of the union to manage its own internal affairs."

At the time of the government challenge. Local 3489 had 660 members, of whom 23 - nine of them incumbent officers - were eligible to hold office.

One defense of the attendance requirements made by the international was that it encouraged local members to turn out at meetings. Disagreeing, the Supreme Court said the average meeting of Local 3489 attracted one in 14 members.

Another defense of the rule was that it assured better qualified officers by limiting candidates to those with a proven interest in union affairs. The court said this runs counter to the congressional intent of leaving the rank-and-file "unfettered by arbitrary exclusions" in elections.