Leaders of organized labor gathered here today, seemingly intent on moving up the date of their endorsement of a Democratic presidential candidate.

Shifting the endorsement from mid-December to early October has been pressed by supporters of former vice president Walter F. Mondale over the objections of two major rivals, Sen. John Glenn (Ohio) and Sen. Alan Cranston (Calif.).

"Nothing can stop it," said William Olwell, political director of the United Food and Commercial Workers, largest of the AFL-CIO affiliates, after a four-hour closed-door meeting with other political directors. "There clearly is a consensus for Mondale, and once there is a consensus, it's better to go ahead."

John Perkins, director of the AFL-CIO Committee on Political Education, said that staff members did not discuss the endorsement date at today's meeting but that it will be settled when members of the federation's executive board meet here Tuesday morning.

Perkins confirmed that when 17 of the large unions' political directors met last Monday in Washington, D.C., they agreed to inform their union presidents that "We need as much time as possible" to organize delegate-selection and get-out-the-vote efforts for the endorsed candidate.

Representatives of Glenn and Cranston, hovering at the edges of the meeting, expressed little optimism about the chances of having the decision left until December.

Mondale has been emphasizing the need for early action in meetings with union representatives this past week. His rivals say he is pressing the issue because he fears slippage in his campaign.

But one well-informed union leader said Glenn has gained almost no advocates among the labor chieftains, despite his recent improvement in the polls. Cranston's appeal to the Food and Commercial Workers convention in Montreal to hold off endorsement until December also fell on deaf ears.

In addition to arguing the need for an early start on labor's political mobilization, Mondale reportedly has told union officials it would "look better to the press" if the endorsement were decided by the full AFL-CIO convention, in open session, the first week in October, rather than in a closed-door meeting of the federation's general board.

The general board consists of about 95 union presidents, with the heads of the 50 state labor federations and some large labor city federations sometimes sitting in.

Federation officials said that if the executive committee decides Tuesday to advance the date, the endorsement decision definitely will go to convention delegates for action. But they held out the possibility that the general board will make a recommendation to the convention on either Sept. 30 or Oct. 2.

The political action committee of the National Education Association, which is not part of the AFL-CIO, is scheduled to make its recommendation for a presidential endorsement on Sept. 28. Mondale is a strong favorite to win its support.

Meanwhile, reports circulating here suggested that the Rev. Jesse Jackson is likely to become the seventh entrant in the race for the 1984 Democratic presidential nomination.

The civil rights activist left federation officials with whom he met recently persuaded that he will enter the race next month. Pollster Patrick Caddell, here to brief the political directors, said he had a long meeting with Jackson last week in which Jackson discussed his strategy for candidate debates and for seeking votes from Hispanics, women and others in his "rainbow coalition."

"He certainly sounded like a candidate to me," Caddell said. The pollster, who helped Jimmy Carter in both 1976 and 1980, told the political directors that President Reagan can be beaten in 1984 only if the Democrats "challenge him directly on his fundamental program, not just disagree at the margins."

Caddell told reporters he was critical of all the current contenders because "no one is making the visionary case against Ronald Reagan."