A suit to overturn a rule prohibiting outside contributions to union election campaigns has been filed against the United Steelworkers.

The complaint in the U.S. District Court for the District of Columbia argues that the union's rule violates First Amendment guarantees of freedom of speech and association, and contravenes federal laws enacted to protect union democracy.

Two of the five plaintiffs in the suit, Edward Sadlowski Jr. and Joseph Samargia, are probable candidates for high office in the union's next election May 28, 1981.

The three other plaintiffs are Sadlowski's father, Edward Sr., a former USWA member, Leonard S. Rubenstein, a district resident and James Miller, a Massachusetts lawyer. All three supported the younger Sadlowski in his successful 1979 bid for the USWA presidency.

That bitter race pitted Sadlowski against current union president Lloyd McBride.It also generated the legal action.

Sadlowski's candidacy was made possible largely through financial contributions and services provided by nonunion members. McBride, who had the backing of the union's leadership, received most of his campaign money from the union staff and hierarchy.

The rule prohibiting outside countributions was adopted after the election.

McBride argued that such contributions would have made the union "beholden to outside interests" had Sadlowski won.

But Sadlowski and his supporters contended that the rule -- actually a complex set of regulations -- virtually would assure election of incumbent union officials or their hand-picked successors.

Labor Secretary Ray Marshall disagreed with that contention. He sanctioned the union rule.

Marshall has also been named a defendent in the suit, filed by veteran civil rights attorney Joseph L. Rauh Jr. along with the Washington law firm of Yablonski, Both and Edelman.

Rauh, who also supported Sadlowski's 1977 race, said the "whole concept of union democracy is at stake" in the suit.

In the petition he said, "If the 'outsider rule' remains in full force and effect, it is unlikely that any candidate will be able to mount an effective challenge against any 'official family' (leadership-backed) candidates in the 1981 election."