Maryland's largest labor group, the state AFL-CIO, announced today that it will not endorse anyone for governor until after the September primary, a move that was seen as a slap at Gov. Harry Hughes, who in the last few weeks has been courting the unions.

AFL-CIO leaders, at a press conference here, said they opted against an endorsement because all three major candidates for governor -- Hughes, Democratic State Sen. Harry J. McGuirk and Republican County Executive Robert A. Pascal -- have good labor records and the group did not want to choose one over the other.

"We cannot see spending money to defeat a friend, particularly in this day and age when it's hard to raise money," said Thomas M. Bradley, president of the Maryland-D.C. AFL-CIO, which represents some 419,000 members in Maryland.

The decision means that neither Hughes or his opponents will appear on AFL-CIO sample primary ballots sent to union members. It also means that no candidate will receive financial contributions or get volunteers or telephone banks from the union for the Sept. 14 primary. Bradley said the 470 locals in his group are free to make individual primary endorsements but he didn't think they would.

The union's decision, the first major event in the election-year scramble for political endorsements, was made more than two weeks ago but kept unofficial until yesterday to allow the labor leaders a chance to evaluate the actions of various candidates during the recent special legislative session on unemployment benefits.

Hughes had been told of the decision when it was made and members of his reelection campaign carefully had put out the word that they anticipated only a general election endorsement.

Four years ago, the AFL-CIO supported Baltimore County Executive Ted Venetoulis in the Democratic primary, but switched to Hughes after he won the primary.

The timing of today's announcement nonetheless gave Hughes' opponents a chance to claim a victory, since it came less than a week after Hughes convened a special session of the legislature at labor's request and signed an emergency measure to deal with a cutoff of federal unemployment benefits.

Hughes also has come out recently in support of a new state labor department, a goal long sought by the unions.

"Obviously they think there's another choice around," Pascal said of the decision. "The man Hughes has not done anything in three and a half years for them or anyone else. You're not talking to a group of naive people."

Several factors were cited for the decision. An endorsement of Hughes now might result in labor issues being ignored for the rest of the governor's race; McGuirk, whose powerful Senate Economic Affairs Committee considers all labor bills, has been helpful to the unions (although others felt he has sometimes secretly wooed business); and Pascal, as Anne Arundel county executive, is still in a position to influence decisions important to labor.

In addition, two long-festering grievances were working against a Hughes endorsement. The American Federal of State, County and Municipal Employes (AFSCME) believes Hughes "double-crossed us," in the words of leader Ernie Crofoot, because he endorsed collective bargaining for public employes as a candidate in 1978 but helped defeat such a bill once elected. (Hughes and Pascal say they favor collective bargaining but are opposed to granting public employes the right to strike or to binding arbitration; McGuirk supports binding arbitration.) And leaders of the Building Trades and Construction Council were upset that Hughes did not intervene to raise wages paid for construction of a state prison in Hagerstown.

Despite these concerns and a major push by Pascal to win labor votes, officials in the major AFL-CIO locals agreed that Hughes is likely to receive the union's endorsement if he wins the primary, as most people expect.

"I think there's already a consensus that we will endorse Hughes in the general election," said one labor official. "They're whistling past graveyards if they think Bob Pascal is going to be endorsed by the AFL-CIO."

Many of the labor officials say that while they like Pascal, his party affiliation has all but ensured that he will not be endorsed. "If Pascal is governor of this state, and we're trying to unseat the president (in 1984 national elections), who's gonna help the president? Bob Pascal. He's got to," one of them said. They also feel that Pascal's lieutenant governor running mate, Newton Steers, is anti-labor.

In addition, the AFL-CIO has made the reelection of Democratic Sen. Paul Sarbanes its prime concern in the November election and labor leaders say an endorsement of Pascal might encourage voters to support Sarbanes' likely Republican opponent, Lawrence Hogan, the Prince George's county executive who is viewed by some labor leaders as a union-buster.