The endorsement season for Maryland's 1980 U.S. Senate race got off to an early start yesterday as the delegates at a state AFL-CIO convention voted to give the union's blessing to the liberal Republican incumbent, Charles McC. Mathias.

The vote came just before Mathias told the crowd of Maryland and District of Columbia union delegates at the Shoreham Americana hotel here that "American labor faces a very serious problem" these days when "it is easier to defeat social legislation on Capitol Hill than pass it."

Mathias, who has amassed a 75 percent approval rating from COPE, the union's political action arm, over the past four years, also pointed out his support for such things as the Labor Law Reform Act, which died in Congress last year.

Maryland's senior senator warned the gathering that "labor is going to have to stay on its toes if it is to have many victories in the 96th Congress," although "there is some encouraging evidence that fast, fancy footwork can still deliver the goods."

More than a year before the election, Mathias seems a heavy favorite to retain the Senate seat he has held since 1969. Maryland Democrats are still mulling over the question of who should oppose a popular Republican who captured nearly 70 percent of the vote in the heavily Democratic state in 1974.

Mathias' campaign committee, Marylanders for Mathias, has already raised $20,015 in contributions since its organization in April, much of it through a $125-a-head June fundraiser.

The contributors ranged from conservative hotel executive J. Willard Marriott, who gave $125 to the Mathias campaign, to former arms control negotiator Paul Nitze, another $125 contributor, and Motion Picture Association of American executive Jack Valenti.

Mathias has also received $2,500 each from the political action committees of the Airline Pilots Association International and the United Steelworkers of America, and $500 each from the Horse Industry Committee on Legislation and Taxation and the American Trucking Association.

A number of Maryland Democrats have been mentioned as possible Senate candidates, but most observers believe that potential candidates are delaying their decisions until they know whether Mathias will face a primary challenge from Rep. Robert Bauman, a conservative Eastern Shore Republican.

Most frequently named, among Mathias' possible Democratic challengers, are Baltimore Congresswoman Barbara Mikulski -- whom Mathias defeated in 1974 -- and CBS network news correspondent Marvin Kalb.