L. Bruce Laingen, the diplomat who was the highest ranking U.S. hostage in Iran, is expected to announce today that he has decided not to seek the Republican nomination for the seat held by Sen. Paul S. Sarbanes (D-Maryland), according to sources in the Republican party.

Laingen, 59, has been under pressure in recent months from Maryland Republicans who want him to resign from his post as vice president of the National War College at Fort McNair to seek the senate nomination. Six weeks ago a draft committee was formed to begin outlining a campaign in case Laingen should decide to run.

Two weeks ago, during a breakfast meeting at a downtown Washington hotel, Laingen told six members of the draft committee that unless a Republican Senatorial Committee poll showed him as an overwhelming leader he probably would not enter the race. He said then, according to two people at the meeting, that while he was interested in entering the race, his wife, Penne, did not want him to run.

"He sounded like a man who was itching to run, he wanted to be a U.S. senator," said one of the committee members. "But apparently his wife really didn't enjoy the notoriety during and after the hostage crisis and asked him to stay out."

The breakfast meeting lasted about two hours. Laingen was told by the committee chairman, Baltimore businessman George Wills, that if he entered the race he would start with about $250,000 in campaign pledges.

Results of the Republican poll could not be learned. Laingen has not returned phone calls from the press since the draft committee was announced and could not be reached yesterday for comment. He has scheduled a news conference for this morning in Baltimore.

His expected announcement would be a major disappointment to many Maryland Republicans. Two committed candidates would be left in the race, Prince George's County Executive Lawrence J. Hogan and Montgomery County businessman V. Dallas Merrell. Former U.S. Sen. J. Glenn Beall Jr. is contemplating a candidacy, and Laingen's withdrawal might bring him into the race.

One man who reportedly had pushed a Laingen candidacy is state GOP Chairman Allan Levey. Yesterday, Levey refused comment on the scheduled press conference. "I guess we'll have to wait until tomorrow Tuesday to find out what the ambassador will do," Levey said.

Laingen was U.S. charge d'affaires in the U.S. Embassy in Tehran when it was taken over on Nov. 4, 1979. Throughout the 444-day crisis he was the hostages' link to the outside world, mostly through phone calls he was allowed to make to his wife.

When the hostages returned, Laingen was their spokesman during the celebrations that greeted them. In January, at the urging of Maryland Republicans, he abandoned his independent registration in his native Minnesota to register as a Republican in Maryland. That gave his backers the impetus to form the committee.

Republicans, who are outnumbered 3 to 1 in Maryland, had hoped that Laingen would have the bipartisan appeal necessary to defeat a Democratic incumbent. Sarbanes has been under attack by the National Conservative Political Action Committee (NCPAC) and has responded by stepping up his campaign. He already had raised more than $400,000 by the beginning of 1982. Hogan, by comparison, had raised $40,000 at that time.