A first test of strength for candidates eager to fill Gladys Spellman's congressional seat will occur today when a powerful coalition of Washington area labor unions met to endorse one of three major Democratic contenders in the upcoming special election in Maryland's 5th Congressional District.

The backing of the coalition, which includes ALF-CIO locals throughout the area of the influential Prince George's County police and teachers unions, is considered important because it will be the first endorsement of what's likely to be a brief but intense campaign for the April 7 primary and May 19 special election.

Although primarily a psychological boon in a campaign's opening days, the endorsement also guarantees some election day and telephone-bank help by union members, as well as fundraising efforts by the union on behalf of the endorsed candidate.

As a result Reuben Spellman, husband of the ailing former congresswoman, State Sen. Edward T. Conroy and former Maryland Senate president Steny Hoyer -- the three Domocrats who have announced their intention to run for the 5th Congressional District seat -- have been emphasizing their pro-labor records and pursuing labor's backing through telephone calls and conversations with their union friends and contacts.

"The psychology of the endorsement is the biggest factor," said one Prince George's County political observer. "That's important from a campaign point of view. Once you get by that, though, it will depend on how active the union will be in the election."

All three, as well as several other politicians who have expressed some interest in the seat, will meet with representatives of the labor coalition today for interviews. The full group, which represents some 80,000 workers in Prince George's, will meet tonight to decide which candidate to endorse. Although the group could endorse one of the four Republicans who have expressed an interest in the seat, such a move in unlikely, members of the various unions said, because of the belief that one of the Democrats will best serve labor's interests in Congress.

It is possible that the coalition may decide against endorsing anyone in the primary because two of the candidates -- Hoyer and Conroy -- have similar pro-labor records. The members of the coalitions may not want to "choose among friends," said Robert Peterson, president of the Greater Washington Central Labor Council, which since November has regularly called the coalition together to discuss the Spellman seat.

According to Peterson and others, the labor coalition endorsement became an issue well before the House of Representatives vote last week that declared the Prince George's seat vacant because of Spellman's continuing incapacitation.

Although she officially was still occupying the seat, Conroy, who was not endorsed by the coalition in his illfated November race for U.S. Senate, began asking members of the group to commit their support to him this time should the seat be up for grabs.

Once the seat was declared vacant last week the coalition apparently was prepared to endorse Conroy, who along with Hoyer recently was rated by the coalitiion as the most pro-labor of the prospective candidates. But the coalition held off its endorsement at the request of Hoyer and Reuben Spellman.

Although Spellman has not actively pursued the labor endorsement since then, Hoyer and several of his supporters have been attempting to woo away several votes in the labor coalition that were committed to Conroy before Hoyer declared an interest in the race. Hoyer's supporters also have said that they will press for no endorsement today if it appears the coalition may vote to give its backing to Conroy.