Late one night last week, State Sen. Meyer (Manny) Emanuel Jr. nervously looked up at the electronic note board on the north wall of the State Senate chambers and whispered, "A loss here would be a tremendous loss in prestige for Steny."

As he spoke, the Maryland Senate was voting on the most controversial topic to come before it this session - an increase in the state sales tax from 4 to 5 per cent - and the red, or "no" votes, outnumbered the green ones.

Senate President Steny H. Hoyer (D. Prince George's) who supported the tax increase, had lost a similar test of strength the previous week when the Senate balked at passing the state budget.

In a place where politicians are measured by their power and how they use it, this was unheard of. As one senator noted, "You just don't lose on something like a budget when your party has a 39 to 8 majority."

This time the Democratic leadership won the sales tax vote by the slimmest of margins, and only after a parliamentary move that outraged opponents.

In the cloakrooms of this city where political reputations are made and lost, the feeling remained that Hoyer's prestige, and with it his campaign for governor, , has slipped in the last three months. A crowded Democratic gubernatorial race is already under way for the September, 1978, primary.

"Hoyer could have come out of this session as a hero, but instead he's going to come out of it as a goat," bluntly declared State Sen. John C. Coolahan, who nominated the Prince George's County Democrat for his post.

"I thought he did a great job last year, but he's been a disappointment as far as I'm concerned this year. He hasn't exerted the leadership he should have," adds Coolahan, who has become Hoyer's nemesis on a series of key issues. "If you're going to be a leader, you have to be a bastard once in awhile. You have to crack heads. He's been afraid of doing that."

Republicans couldn't be more pleased. "Up to this time, Steny has been the rising star around here, but his star has peaked," said Senate minority leader Edward J. Mason of Cumberland. "The real problem is you can't control the Senate and run for governor at the same time. He's let things slip out of his hands. The Senate has become less formal. It's become almost rowdy on the floor."

Hoyer is also criticized for not pushing hard enough for budget cuts and proposing a tax package that included a $10 per taxpayer income tax rebate.

It is far too early to judge Hoyer as a contender for the Democratic nomination. For one thing, he is partially the victim of circumstance and the weak cast of supporting characters in the Senate Democratic leadership.

More important, he has a strong political base in Prince George's County, a legislative record with strong appeal to consumer environmental and other public interest groups, and a good reputation for leadership in previous Senate sessions.

"It's like a fighter you see getting hit in the ring," said Sen. Harry McGulk, a powerful Baltimore Democrat. "He's been hit hard, but it's a long time until the end of the round."

Hoyer, 38, left little doubt that he hoped to jump to an early lead in the gubernatorial sweepstakes when the General Assembly opened in January. He carefully orchestrated the opening of the Senate, recruiting former opponents like Coolahan to speak for him. In an unusual state of the state address, he issued a legislative package and struck a note of independence from Gov. Marvin Mandel.

The effort was precise and methodical. Its aim - a show of force that would impress his opponents and the people who finance million dollar campaigns. If Hoyer could make a good impression during the three-month legislative session, the word would "filter back into the community," his close friend Peter F. O'Malley, the leader of the powerful Democratic organization in Prince George's County, said at the time.

Ironically, it was O'Malley's push for a $35 million convention center in Prince George's County that may have caused Hoyer his biggest setback.

Hoyer took no visible part in the unsuccessful lobbying for a $20 million state bond issue to finance the center. O'Malley, his campaign manager, and best friend originated the controversial deal to build the center intermixing public and private money without "Hoyer's knowledge, Hoyer said.

But the Hoyer-O'Malley connection was impossible to ignore. "Mr. O'Malley's timing was awful, for it cast him in the role of political manipulator, not the civic minded organizer Mr. Hoyer likes to think of him," said the Baltimore Sun in an unusually biting editorial. ". . . His relationship with Mr. Hoyer is bound to be an issue in next year's gubernatorial campaign."

Some of Hoyer's Senate colleagues are even harsher. "The convention center thing stinks," said one senator, who asked not to be identified. "It makes Steny look like a wheel-dealer and not an effective one at that," said another Hoyer critic.

Hoyer is deeply troubled by such attacks, and somewhat baffled about how to counter them. In a midnight interview after the sales tax vote, the usually composed Senate president snapped at a reporter when asked about the convention center. His face grew barn red and at one point his eyes began to water.