Stepping into a regulatory and judicial fight between competing telecommunications interests, a Republican Senate coalition yesterday introduced legislation that would modify a 1956 consent decree between American Telephone & Telegraph Co. and the government and permit AT&T to offer unregulated services.

The legislation, introduced by Sen. Robert Packwood (R-Ore.), chairman of the Senate Commerce Committee, and four other Republican committee members, is likely to touch off intense debate between AT&T, competitors like MCI Communications Corp. and computer concerns opposed to legislative modification of the landmark consent decree.

Although the bill contains language asserting that the legislation is not intended to affect ongoing antitrust cases, such as the Justice Department's case against AT&T, the decree modification may raise questions about that move, which the Justice Department historically has opposed.

The legislation also would put Congress into the middle of a debate before a New Jersey court responsible for the decree concerning the Federal Communications Commission's authority to modify the Bell System structure despite the decree's language.

Noting that court fight, Packwood said that "in light of these events, we believe it is important" to proceed with the legislation and schedule hearings as soon as possible.

In effect, the legislation adopts pieces of the FCC's "Computer II" decision, which forces AT&T to set up a separate subsidiary to offer competitive services, such as data processing.

But, according to a summary releaded by Packwood, the legislation would also bar AT&T from offering "mass media" or cable-television-type services through transmission facilities owned or controlled by the company or its affiliates.

However, AT&T could provide electronic Yellow Pages, weather, time and sports information but not printed or electronic publications, such as newspapers, the bill summary said.

An AT&T spokesman said he is "pleased that the bill has been introduced because it is an indication of congressional interest and shows that Congress is stepping up to its responsibilities.

"We firmly believe that legislation which will establish the rules for everyone to abide by is in the public interest and should be adopted," said the spokesman.