The AFL-CIO may break with its past political practices by endorsing, early, a single candidate for the Democratic presidential nomination. But that doesn't guarantee that the union will hand over a large chunk of votes to the candidate it backs in the 1984 election.
Lane Kirkland, president of the 95-union federation, has called for labor solidarity behind a presidental candidate, saying, ''We have learned the hard way that to divorce ourselves from the selection process may sometimes deprive us of a clear and meaningful choice in the voting booth.''
But he concedes that labor leaders have ''no power and no wish'' to dictate how millions of union members will vote in 1984. AFL-CIO and its affiliates can make no claim, Mr. Kirkland says, to an ability to ''control or deliver'' votes.
A decision on an early endorsement is expected to be made by AFL-CIO's general board of the heads of all the affiliated unions, who meet in Hollywood, Fla., on Saturday. A two-thirds vote will be necessary. There is opposition, and debate could become heated, but approval appears likely.
If there is a quick endorsement, former Vice-President Walter F. Mondale almost certainly will become the AFL-CIO's candidate. His strongest challenger, Sen. John Glenn (D) of Ohio, already is preparing to challenge such an endorsement in a strong fight for workers' votes in state primaries and the Democratic convention next year.
Senator Glenn recently wrote the presidents of AFL-CIO unions, suggesting that labor would be better served by devoting political time and money against President Reagan instead of for the selection of a single Democratic candidate. A number of national polls have shown Glenn to be as popular as Mr. Mondale in union households.
Sensitive about the possibility of charges that a single, early endorsement would not be democratic or wise, many unions within AFL-CIO have polled members on whether to support a change from the federation's past policy of waiting until the national conventions for a presidential endorsement. No results have been announced, but some political strategists have acknowledged ''disturbing opposition'' to a change.
Kirkland says that greater influence in the choice of a presidential candidate that all workers can support would be an important first step toward such ends. He warns that labor cannot again ''enter the candidate selection process - the presidential primaries, caucuses, and party convention - with our forces fragmented and divided.''
Critics of the AFL-CIO early endorsement plan will argue on Saturday and at the federation's biannual convention opening two days later that a premature endorsement could prove to be ''a dreadful miscalculation'' by tying AFL-CIO to a candidate who makes a weak showing in the states.
Meanwhile, the president of the International Brotherhood of Teamsters, Jackie Presser, said over the weekend that the IBT, the country's largest union, would support President Reagan for reelection in 1984 if he is opposed by Mr. Mondale. The union backed Reagan in 1980. Despite disagreements with him from time to time, Mr. Presser said the 1.8 million-member IBT ''remains committed to him. . . . If it comes down to him vs. Mondale, yeah, it's Reagan.''
Another major independent union, the broad-based National Education Association, is likely to endorse Mondale on Sept. 29. A number of state NEA affiliates, including California's, may abstain from the early voting.