Leaders of organized labor are fundamentally misjudging the electorate's mood if, as expected, they endorse the presidential candidacy of former vice president Walter F. Mondale, according to Mondale's chief rival for the Democratic nomination, Sen. John Glenn (D-Ohio).

Arguing that opinion polls show him to be a stronger candidate against President Reagan, Glenn said in a weekend interview that the AFL-CIO would be pursuing a short-sighted strategy by putting its resources behind Mondale.

"It doesn't do them any good just to win the nomination," Glenn said. "That's not the objective, unless you're just looking at this for a demonstration to show you can get your nominee in.

"The objective that is going to benefit their membership is going to be who can replace Reagan in the White House. That's what we have to keep in mind. In that regard, the polls have very consistently shown me to be ahead in that department," he said.

Glenn said he has not abandoned hope of obtaining labor's endorsement but spoke as if he expects that it will go to Mondale. The AFL-CIO plans to announce its choice in early October.

Most political experts assume Mondale will obtain labor's backing.

"If they go that way, do I think they're making a mistake?" Glenn asked during an interview between campaign stops. "Absolutely. It would just make it a little more difficult for me to overcome that additional help he Mondale would have. It means I would have to raise more money.

"Would it make it impossible? No."

As he has before, Glenn expressed confidence that he can compete successfully with Mondale for the votes of rank-and-file workers during next year's Democratic primaries and caucuses.

"The fact that there's an endorsement doesn't necessarily mean all their membership will drop in line and you'd then have a huge voting block you would have to overcome," he said. "I think the voting is going to be pretty well split up whatever the endorsement of leadership is."

Glenn acknowledged that the resources, potentially including $10 million to $20 million in services, that will accompany the endorsement would be of enormous value to Mondale.

"I'm not underplaying the endorsement ," he said. "It carries a lot of money along with it, phone banks and . . . all the other things that are important. So it's an important thing."

But he said that labor will hurt itself as a political force if its candidate fails to win the Democratic nomination, and again predicted that he is in the best position to win.

Asked if he feels labor is misjudging the nation's mood by turning toward Mondale, Glenn replied, "Obviously, I think so, because I certainly plan to be the nominee and plan to be elected."

In that assessment, Glenn seemed to be expressing self-confidence about his campaign rather than attacking Mondale as a candidate who is not electable.

Glenn said that while he does not see the labor endorsement as a fatal flaw to a candidate in the general election, "the biggest advantage is in the nominating process."

Glenn's campaign advisers have said that, if Mondale wins the labor endorsement, they will not run an anti-union campaign against him.