The AFL-CIO executive council yesterday unanimously approved an ambitious plan to throw the weight of organized labor behind a consensus 1984 presidential candidate by mid-December, 1983.

About 500 labor leaders, their votes weighted to reflect the size of their unions, will vote on the endorsement at a mini-political convention of the AFL-CIO general board in early December. This schedule was dictated by the filing deadlines for delegates in various states, labor federation president Lane Kirkland told reporters at a news conference.

To win the labor endorsement, a candidate must get the votes of the delegates representing two-thirds of the 15 million union members at the gathering. The efforts of the Democratic presidential hopefuls to gain the endorsement, or at least win enough support to deny anyone else the two-thirds majority, could substantially alter the presidential campaign next year.

Though Kirkland acknowledged that the obstacles confronting labor leaders in this effort are considerable, "There has emerged in the movement a general craving for unity of action," he said.

The new approach is the result of a "history of groping" for an effective formula in the face of radically changed presidential delegate selection rules since the so-called "McGovern reforms," Kirkland said.

The scattershot strategy in the past two or three presidential elections in which labor split between such presidential candidates as Jimmy Carter and Sen. Edward M. Kennedy (D-Mass.) "didn't work out," he said.

The AFL-CIO may decide, for any of a number of reasons, not to endorse anyone, or to defer the endorsement, but Kirkland said that such a course likely would return them to the ineffectual strategies of the recent past.

"Even in the event of an endorsement by the AFL-CIO, the affiliated unions are constitutionally free to follow a different course," Kirkland noted. "The considerations that are brought to bear on them in the event of an endorsement are moral, and I do believe that there is a spirit now amongst our affiliates whereby they will be quite likely to support" the organization's candidate.

The leaders of a number of key unions, including the machinists and the huge American Federation of State, County and Municipal Employes (AFSCME), which endorsed Kennedy in 1980, have indicated they will support the chosen candidate even if he is not their own first choice.

The AFL-CIO might decide not to endorse any candidate before the primaries, Kirkland said, because of the absence of a consensus among the local unions, or a perception that labor would be satisfied with any of the likely candidates and "there is no way we could really lose."

The campaigning is already under way, with two presidential contenders appearing before the federation's executive council here this week.

Sen. John Glenn (D-Ohio) was today's featured guest. The labor chieftains gave him high marks, as they had Sen. Gary Hart (D-Colo.) earlier in the week.

The endorsement plan has prompted mutterings in some quarters about the revival of the old days of the smoke-filled room.

Asked if that was a possible negative factor for a candidate, Glenn replied, "The day of labor bosses controlling a block of votes is past." The average union member is better educated than in the old days when that was effective, he said.

Kennedy and former vice president Walter F. Mondale are perceived as the most likely to win the endorsement.

The federation plans to invite all other competing candidates to appear before them for consideration at future gatherings, according to spokesman Murray Seeger, including Republicans and those with poor records on labor issues.

If there is competition for the Republican nomination, Kirkland said, there is a possibility the federation will endorse one candidate from each party.