The Reagan administration yesterday offered its first suggestion for broadening rather than cutting back the Freedom of Information Act.

It suggested that Congress reconsider its unwillingness to put its own files and records within the reach of the law.

"Nothing in our review of the act to date has convinced us of the wisdom or necessity for this complete and total congressional exclusion," assistant attorney general JOHNATHAN C. Rose told the Senate Judiciary subcommittee that has jurisdiction over the measure.

The subcommittee, headed by Sen. ORRIN G. Hatch (R-Utah), opened what is expected to be a long-running series of hearings aimed by and large at cutting back the disclosures required under the law since 1974 when Congress first put teeth into it.

Rose made clear that the administration favors adoption of restictions on FOIA, which he charged has "significantly impaired the investigatory abilities" of the FBI and other federal law enforcement agencies as well as the ability of the CIA and other U.S. intelligence agencies to gather confidential information.

But he urged at the same time that Congress re-examine the total exemption that enables it to keep its records secret for as long as it chooses. The FOIA applies only to executive branch agencies.

"Certainly no body of the federal government has more to do with how key decisions affecting our citizens are made," Rose testified. "Why, then, should the files of Congress be totally exempt?

Rose, the head of Justice Department's Office of Legal Policy, said there was no comparable need to subject the courts to the act "since the judiciary operates on a public record."

Rose said the Justice Department is still developing a set of proposed amendments to the FOIA on the basis of comments solicited since springtime from all government agencies. He predicted that a comprehensive package would be ready for introduction within the next two months.

Contending that the FOIA must be tested against the standard of "effective government" as well as "open government," Rose also argued that the law should be changed to require higher fees, especially of commercial users, and that amendments are needed to prevent disclsure of confidential business records.

Hatch said in an opening statement that he was all for open government, but he also said he believes the law has "occasionally disrupted vital national security and law enforcement activities" and been misused by businesses seeking information about their competitors.

Other witnesses at the Senate hearing included spokesmen for the Society of Professional Journalists who urged Hatch not to use "crowbars" on the law when screwdrivers would do. Washington correspondent STEVEN R. Dornfeld, one of the spokesman, also asked that complaints of the FBI and other law enforcement agencies about their informants being afraid of inadvertent disclosure under FOIA be taken wtih a grain of salt. He suggested that many of these fears are fostered by the law enforcement agencies themselves because they don't like the FOIA.

The House Government Information subcommittee held an FOIA oversight hearing at the same time yesterday, but its chairman, Rep. Glenn English (D-Okla.), said he feels those seeking amendments must "bear the burden of providing adequate justification."

Witnesses at the House hearing ranged from the National Association of Manufacturers, which complained of a "decidedly pro-disclosure posture" at some government agencies, to Ralph Nader who argued that the existing law and pertinent court decisions provide business interests with ample protection against improper releases.

Nader also argued that government complaints of the costs of FOIA, estimated at more than $45 million a year, overlook how much money is spent on "Multiple layers of review . . . stalling tactacs . . . [and] needless litigation."