Maryland Senate President Steny H. Hoyer has decided to drop his campaign for governor and accept a spot as the lieutenant governor candidate on a ticket headed by Acting Gov. Blair Lee III in the September Democratic primary, authoritative sources said yesterday.

Hoyer is expected to withdraw as a candidate for governor today in a series of meetings with his supporters and at a Baltimore press conference. Lee has scheduled a press conference of his own for Thursday, at which he plans to introduce Hoyer as his running mate, according to several sources.

The Lee-Hoyer political marriage, which has been anticipated for several weeks, will dramatically change the shape of the Democratic primary. Hoyer, an energetic 38-year-old legislator, brings to the Lee camp a strong core of support within the General Assembly and, more important, the backing of the powerful Democratic organization in his home county, Prince George's.

With Lee, a Montgomery County resident, picking up Hoyer as a running mate, the acting governor will field a ticket made up exclusively of Washington suburban politicians. The ticket is expected to free the four Baltimore-area gubernatorial hopefuls to select running mates from their home base if they choose to continue their campaigns.

The other candidates, all of whom will now be reevaluating their campaigns, are Attorney General Francis Bill Burch, Baltimore City Council President Walter S. Orlinsky, former state transportation secretary Harry R. Hughes and Baltimore County Executive Ted Venetoulis.

Rumors that Hoyer would drop back and join Lee's ticket have been floating around the state's political network for several months. They became more persistent in recent weeks as Hoyer continued to show a low rating in statewide polls and as the two candidates and their advisers began meeting on a regular basis.

At the last such negotiating session, in the middle of last week, Lee reportedly made a final offer to Hoyer, telling him to decide whether he would accept or reject the lieutenant governor's spot by the end of the Memorial Day weekend.

Hoyer appeared drawn and pensive over the weekend as he considered his political future, his friends sol. He continued campaigning up to the last minute, running a series of television advertisements during the weekend and attending several ceremonies and events on the Eastern Shore.

Then, late Sunday night, he sat down on a couch in the recreation room of his home in District Heights and went over his alternative with his two closest advisers - his wife Judy and Prince George's attorney Peter F. O'Malley.

"It was on opportunity for Steny to think out loud and recite the conclusions he was developing," O'Malley said. "I don't know whether he had completely resolved it in his own mind that night, but I had a pretty good idea what he was going to do by the time I left three hours later."

O'Malley said Hoyer asked him what he should do, and the longtime friend replied: "Whatever you think is best."

At noon yesterday, O'Malley said, Hoyer called him and revealed his decision, which O'Malley refused to comment on.

O'Malley apparently was one of a select handful confidantes told of Hoyer's decision. Several other Prince George's Democratic leaders - including County Executive Winfield Kelly Jr. - said yesterday they knew only that Hoyer had made up his mind.

"I guess we'll learn what Steny will do tomorrow (at the press conference), along with the rest of you," said state Sen. Thomas V. (Mike) Miller.

For Hoyer, the decision to withdraw from the gubernatorial primary brings to an end a campaign that began more than two years ago in a late night strategy session at his home.

Although several friends advised him then that he would be better off seeking re-election to his Senate seat and waiting for another four years to run for governor or U.S. Senate, Hoyer decided 1978 was his year.

"Steny is wrapped very tight," explained one longtime friend. "He wants to be governor some day, he wants to be a U.S. senator some day, and he wants to be president some day. If he doesn't make it this year to governor, it may delay him, but I don't think it will stop him."

What apparently stopped Hoyer this year were the political polls. Despite two years of personal campaigning in every corner of the state, he failed to boost his recognition level above 10 per cent of the Democratic electorate in any poll.

In a poll published Sunday in the Baltimore Sun, hours before his decisive meeting with his wife and O'Malley, Hoyer received the backing of just 6 percent of the Democrats canvassed, trailing far behind front runner Lee, who held the support of 24 percent.

According to the poll, Hoyer won substantial support only in Prince George's County, but even there Lee demonstrated more appeal than Hoyer.

In recent weeks, as Hoyer's departure from the race appeared imminent, his supporters began offering various ideas on what his next step should be. Some, like Kelly, said Hoyer would be wise to return to the Senate leadership post he has held since 1975.

Others suggested that if Hoyer joined Lee he could become the most powerful lieutenant governor in Maryland history. There was a consensus among his supporters that, if he dropped out to become a lieutenand governor candidate, it should be on Lee's ticket.

"Lee's a softer spoken man than the other candidates, said state Sen. Melvin A. Steinberg (D-Baltimore County). "Steny would be able to blossom under him.Francis Burch isn't his kind of candidate. And Ted Venetoulis would keep him in the shadow all the time."