Campaign operatives of Sen. John Glenn (D-Ohio) are trying to throw a monkey wrench into the AFL-CIO's expected endorsement next month of former vice president Walter F. Mondale, his chief rival for the 1984 Democratic presidential nomination.

While the other Democratic contenders have conceded the endorsement to Mondale, Glenn's strategists are trying to persuade a number of key union presidents to split their unions' votes instead of casting them all for one candidate, as some have indicated they would do.

The leaders of the 98 AFL-CIO unions will cast a total of 14.5 million votes on behalf of their membership when the AFL-CIO general board meets in Hollywood, Fla., on Oct. 1. Each president will cast a number of votes equal to his union's membership.

To win the AFL-CIO endorsement a candidate must get 9.8 million votes, or two-thirds of the total.

"Some unions are talking about splitting their votes, and we are encouraging that," said Glenn aide Jack Dover.

He refused, however, to speculate about Glenn's chances of blocking the endorsement of Mondale, which are widely considered to be slim. Dover could not name any major unions that have committed to a split vote and some labor sources privately expressed dismay at the idea.

But Dover cited a rash of summer polls showing the former astronaut's popularity with the rank and file. And he said Glenn can count on active blocks of support among the United Auto Workers and the United Steelworkers, particularly in his home state of Ohio.

The campaign is also looking for help from Glenn backers in the Food and Commercial Workers, Plumbers, Brotherhood of Electrical Workers, Letter Carriers, Postal Workers and Seafarers unions, he said. The Firefighters and Paperworkers unions are counted as solidly supporting Glenn, but can supply only a few hundred thousand votes.

"We have a lot of labor support," Dover said. "Whether that goes along with the downtown AFL-CIO headquarters interest in a monolithic candidacy, I don't know."

Even if Glenn fails to deny Mondale the endorsement, his strategy might thwart the federation's hope of presenting, for a change, a united front on behalf of its chosen candidate. And his aides are convinced, they say, that he would come away with a showing of substantial labor support.

"I think those who like Glenn want to show support for him," said Martin J. Hughes, an official of the Communications Workers of America in Ohio who is generally recognized as Glenn's point man in organized labor.

He has no commitment from CWA President Glenn Watts to split that union's vote, he said, but added, "I'll be talking to him about it . . . . I think union leaders are reluctant to make a commitment on this yet."

Each union has established its own procedures. Some have taken polls that provided a detailed percentage breakdown of the membership's candidate preference. In these unions, a president might cast a percentage of votes for each candidate based on the poll results, labor insiders speculated. Other unions have taken no such votes, but have developed a "sense of the membership" through meetings and other contacts.

In his official Labor Day statement, Glenn signaled his intentions with an implicit appeal to the rank and file to challenge their leadership regarding the AFL-CIO endorsement.

AFL-CIO President Lane Kirkland has already told the union presidents that they have the option of splitting their votes if they wish, "to show that this is a democratic organization," said spokesman Murray Seeger.