With less than three weeks left in the presidential campaign, the AFL-CIO is intensifying its nationwide efforts to convince millions of workers that Walter Mondale can defeat President Reagan on Nov. 6.
The implications are large for the labor federation. It risked an early endorsement of the former vice-president - considered a good friend by unions - before important state primaries and caucuses. And it has backed him to the hilt throughout months of campaigning.
A Reagan victory next month, particularly one giving him a substantial margin over Mr. Mondale, could cause deep-reaching problems for the AFL-CIO and its president, Lane Kirkland. A big Republican win would foster internal doubts about the federation's leadership and policies, and further reduce public and employer estimates of labor strength in politics and in the marketplace.
Although there has been a grim awareness of this possiblity, there recently has been some optimism among AFL-CIO leadership that a surge of labor support for Mondale can ''transform the presidential election'' in the campaign's closing days.
Mr. Kirkland is well into a nine-state tour meant to capitalize on what appears to be resurgent worker support for Mr. Mondale.
After campaigning the Northeast in a camper dubbed the ''Solidarity Van,'' the AFL-CIO president and his aides moved into the Midwest on Thursday, carrying the message: Mondale can win.
At the same time, Kirkland is making a strong pitch for Democrats running for Senate and House seats in an effort, he says, to end ''effective control of Congress by a conservative bloc.''
Federation spokesmen, including John Perkins, director of the AFL-CIO's Committee on Political Education (COPE), report a changed attitude among union members and their families toward Mondale and Geraldine Ferraro. The change has occurred since the first two nationally televised debates sponsored by the League of Women Voters.
The Reagan-Mondale debate adds an ''immediately visible impact'' to the campaign, Mr. Perkins says. While AFL-CIO polls and the union's reading of other national polls show President Reagan still ahead in the presidential race, the margin of the lead has been cut.
Labor politicians are now talking about ''another Harry Truman miracle'' in reference to Mr. Truman's come-from-behind victory over Thomas Dewey in 1948.
Before the Reagan-Mondale debate, organized labor was deeply pessimistic about Mondale's chances. According to a New York Times/CBS News poll of union members in mid-September, the members were about evenly split between the candidates, with 44 percent for Ronald Reagan and 43 percent for Walter Mondale.
In the week before the debate, the same poll showed a sharp gain for Mondale, that gave him a 48 percent to 35 percent lead. Now labor analysts say the Democratic candidate appears to have forged further ahead among union voters. The gloom is dissipating.
However, Kirkland said earlier this year that it would take a 65 percent majority of labor voters to ensure Mondale a victory. He was confident, at the time, about mobilizing that amount of support.
Although it will be difficult for union leaders to muster the predicted level of support, if they fail, it will not be for a lack of effort.
Kirkland is scheduled for rallies and speeches in Pittsburgh, in Akron, Cleveland, Sandusky, and Toledo, Ohio, and in Detroit, where he will be joined by United Automobile Workers leaders. He will go on to Lansing and Grand Rapids, Mich., then to Chicago, Rock Island, and Rockford, Ill., and finally to Kenosha and Milwaukee, Wis. In addition to the 13 cities in eight days, Kirkland is expected to ''stump a little'' wherever there are industrial workers along the route.
Presidents of AFL-CIO affiliated unions are also campaigning hard. Prominent among them is Lynn Williams of the United Steelworkers, who ''senses a turnaround'' in Mondale's favor. Perkins, of COPE, says volunteers all over the country are working harder than ever now.
Whether the late drive will be successful depends on how deep the turnaround is in the unions. While President Reagan appears to have lost some labor support , and Mr. Mondale's figures have risen a bit, large numbers in the unions' rank-and-file memberships apparently have not definitely decided between Reagan's claim of economic gains over four years and Mondale's promises for the future.
Many seemed to be waiting for yesterday evening's second Reagan-Mondale debate. But neither side can count yet on winning the labor vote.