Former Maryland Senate President Steny Hoyer won a key endorsement last night in his bid to fill Gladys Spellman's congressional seat when a powerful coalition of Washington and Baltimore-area labor unions voted to back his candidacy over that of several other Democrats.

The decision by the labor coalition, which included members of AFL-CIO locals throughout the area and the Prince George's County police and teachers unions, was a significant victory for Hoyer and a defeat for one of his strongest Democratic opponents, State Sen. Edward Conroy, who late last week appeared certain to win the group's backing.

According to associates of the Bowie senator, Conroy had considered the labor endorsement essential to getting his still-sluggish campaign for the seat moving. The associates of Conroy said he has been unable as yet to convince longtime campaign organizers to commit themselves to this race and was expecting the labor endorsement to launch the congressional campaign. Conroy was unavailable for comment last night.

In giving their backing to Hoyer, the labor groups also overlooked requests for support in the April 7 special primary and May 19 general election from Reuben Spellman, Gladys Spellman's husband, as well as several other Democrats who have expressed an interest in filling the fifth congressional district seat.

The endorsement last night, which must be formerly approved later this week by the various unions in the coalition, will provide Hoyer with several important campaign assets, including campaign workers, telephone banks and campaign contributions, in a district that is heavily dominated by federal and state workers. The endorsement also provides him a psychological lift in the opening days of his campaign.

Hoyer said last night the endorsement was a "tremendous boost" and added that he hoped Conroy, who has not yet filed to be a candidate in the race, would reconsider running and instead back Hoyer.

According to labor leaders at the meeting, the group decided to support Hoyer only after a protracted debate between his supporters in the coalition and those backing Conroy. Hoyer was chosen, one labor official said, because the group felt he had a better shot at winning in the special general election; against a Republican candidate who is likely to be well-financed with the help of the Republican National Committee. The group was apparently concerned that a Conroy race would be hindered by the debts he still has from his recent unsuccessful challenge against U.S. Sen. Charles McC. Mathias Jr.

In addition, the labor group, which had been heavily lobbied by Hoyer and his supporters in the last few days, felt that Hoyer would have more time to campaign against a Republican than would Conroy, who is now in Annapolis attending the General Assembly session.

Mrs. Spellman, who represented the district since 1975, was stricken with heart arrest Oct. 31. Four days later, while hospitalized and semiconscious, she was overwhelmingly relected. Her seat was declared vacant a week ago after she failed to respond to months of hospital care.