After a summer slump, Walter F. Mondale has gained an autumn advantage and a winter weapon that are the envy of his rivals for the Democratic presidential nomination.

Today's decision by the Executive Council of the AFL-CIO to advance its presidential endorsement decision from December to early October sets up what aides to the former vice president hope will be a "dream week" this fall, establishing Mondale as the clear favorite in the race.

Beyond that, it puts him in line to acquire, well in advance of the first caucuses and primaries, a supplemental organization -- labor's political machine -- which, by itself, is probably more than a match for anything any of his five opponents can put in the field at this point.

This has come to Mondale at a time when gains for Sen. John Glenn (D-Ohio) in public opinion polls have stirred talk of "erosion" of Mondale's position as front-runner among many Democratic Party officials.

William White, manager of Glenn's campaign, made the obvious point today when he said that "the real test [of the candidates] will come when the caucuses and the primaries begin," so any judgment about Mondale "locking up" the nomination is premature.

But Victor Kamber, a spokesman for Sen. Alan Cranston (D-Calif.), another major hopeful, expressed a widespread view when he said that the labor decision "makes it much tougher for any of us to compete with Fritz."

The effect is two-fold: psychological and practical.

The public relations benefits should appear late next month, at the point when most observers think the nomination game will shift from being strictly "inside baseball" chatter for a handful of political activists and start to become a topic of more general public interest.

The schedule is shaping up this way:

On Thursday, Sept. 29, the political action committee of the National Education Association will meet in Washington to recommend a presidential candidate. Almost all his rivals consider that Mondale has that endorsement locked. The NEA board of directors will meet to confirm that recommendation on Friday, Sept. 30.

The next day, Saturday, Oct. 1, Mondale and Cranston will square off in the Maine Democratic convention straw vote.

Cranston has mounted a major challenge in that state, hoping to repeat his upset of Mondale in Wisconsin. But Mondale has decided to cut back his efforts in earlier and later straw votes in New Jersey and Florida to go "all out" in Maine.

Glenn, who is playing down the straw votes, is not considered a major factor in Maine at this point.

On the weekend of Oct. 1-2, the general board of the AFL-CIO will meet in Hollywood, Fla., to recommend an endorsement, presumably of Mondale, to the convention, which opens Monday, Oct. 3. The endorsement vote will come on the convention floor between Oct. 3 and 7.

On Thursday, Oct. 6, all six Democratic candidates are scheduled to be in New York City for a party forum that is expected by all camps to produce a real issues debate. Both Mondale and Glenn strategists said today they expect the candidates to start "defining their differences" that night.

The clear hope of Mondale's campaign is that if he survives the week without any wounds, he will have put his stamp on the public mind as presumptive nominee.

White said the Glenn camp has "no events planned" to offset the anticipated Mondale blitz, but others suggest that the mid-October release of a movie version of "The Right Stuff," the book about Glenn and the other original astronauts, may be a public relations bonanza for the Ohio senator.

Whatever the impact of the October events, there are immense practical advantages awaiting Mondale from labor's expected endorsement.

Jack Dover, Glenn's labor aide, estimated that the manpower and services the AFL-CIO and its affiliated unions can provide a national candidate would cost the candidate $15 million or $20 million to buy. That is about two-thirds of the legal limits a presidential candidate can spend in the primaries, which in effect almost doubles his resources.

While some labor officials consider that estimate high, they brag that they can probably muster 100,000 volunteers, provide thousands of free phones from local headquarters, and give the endorsed candidate the benefit of free mailing from some of the most up-to-date computerized lists of registered voters available.

All of this can be financed from union dues, not voluntary contributions, because it is directed at union members and their families and thus comes in the category of internal communications. Contributions to labor delegates running on the Mondale slates would have to come from voluntary donations, union officials said. Mondale has ruled out accepting cash contributions from labor unions or other interest groups.

The AFL-CIO Committee On Political Education (COPE) has prepared workbooks on the delegate selection process in all 50 states. COPE will hold workshops on each of the 20 largest states, and in groups, for all the other states, before the end of September.

Officials said nine to 11 mailings are planned for voters in targeted states, districts and precincts before their caucuses and primaries. Data from polls that COPE has begun taking for 1984 Senate and House races also will be made available to the endorsed candidate.

Labor's endorsement has been considered a mixed blessing by some observers. A recent Gallup Poll reported that 18 percent of the voters would be more inclined to support a candidate endorsed by labor, but 35 percent would be less inclined.

However, the same poll showed those figures virtually reversed -- 32 percent more inclined and 18 percent less -- in households with union members.