Leaders of some labor unions that favor former vice president Walter F. Mondale for the Democratic presidential nomination are making an effort to advance the AFL-CIO's endorsement decision by two months in order to improve his chances.

Glenn Watts, president of the Communications Workers of America, predicted yesterday that the endorsement date will be advanced from December to October, an action that he said would heighten the importance of labor's blessing.

"Once we have a consensus," Watts said, "there's no reason to wait, and there's every reason to move ahead."

A speedup is supported by such other unions as the American Federation of State, County and Municipal Employees (AFSCME) and the Service Employees International. But it is by no means clear that the push will be successful.

The United Auto Workers have just had a major turnover in leadership. And a spokesman said that it is not ready to choose a candidate.

Building trades unions reportedly are divided not only over a candidate but on the wisdom of a pre- primary endorsement.

AFL-CIO President Lane Kirkland is in Europe for a month. But John Perkins, director of the federation's Committee on Political Education (COPE) said that he is unaware of any proposal to advance the endorsement date.

The matter has been left to an August meeting of the AFL-CIO executive council. And there is broad agreement among the union presidents that Kirkland will be able to use his discretion in determining when to take a vote.

The endorsement decision, an innovation Kirkland has pushed, will require a two-thirds vote of the federation's general board, with each union president casting a single ballot weighted for his membership. It had been understood that the board would meet for that purpose in early Decmember. But Watts said, "It would make more sense to do it when we're all together" for the AFL-CIO convention in Florida the first week in October.

That would make labor's balloting virtually simultaneous with the endorsement decision of the National Education Association that is scheduled for Oct. l.

Mondale, who has been favored to win both nods, would like to get them early, according to his staff.

Richard Murphy, political director of the Service Employees, said that some pro-Mondale unionists are concerned about gains in the public opinion polls by Sen. John Glenn (D-Ohio) and by the recent straw-vote victories of Sen. Alan Cranston (D-Calif). "We might save him Mondale some damage by doing it early," Murphy said.

Cranston reportedly phoned several union presidents after his upset win over Mondale in the Wisconsin straw poll and reiterated his desire for a December decision.

William Holayter, political director of the Machinists said that his union will oppose any effort to move the decision to October. "We want to look at the situation as long as possible," he said. The president of that union, William Winpisinger, is a Cranston admirer.

But AFSCME's Jerry Clark said his union's internal leadership meetings have brought out "a very strong consensus for Mondale," adding that it wants to move as soon as possible.

Individual internationals have been told by Kirkland to make no formal endorsements until after the August executive council meeting.