Lane Kirkland is a quiet man, the kind who is easy to underestimate. There is a touch of the academic in his manner, a bit of the bureaucrat in his speech. Combined with the soft accents of his native South Carolina, it makes him seem a guy easy to muscle around.

But the president of the AFL-CIO is about as tough as they come. At a time when a great many organization Democrats and elected officials are looking for an easy way to beat Ronald Reagan in 1984, Kirkland and Co. have decided to do it the hard way--the way they think is right.

That way is to go with Walter F. Mondale, the committed Democratic partisan and card-carrying liberal, for the presidential nomination, rather than try to ride the popularity of space-hero all-American Sen. John Glenn of Ohio into the Whilte House.

That is the meaning of the decision Kirkland engineered here last week at the AFL- CIO executive council meeting, advancing the date of the labor federation's endorsement decision from mid-December to early October. In an organization that depends on consensus-style decision-making, you don't schedule a vote until you know there is agreement. Kirkland now has the consensus behind the candidate he probably favored all along.

Why Mondale? Why the guy who was vice president in the Democratic administration voted out of office less than three years ago? Why the guy that the summer polls have shown to be a slightly weaker candidate against Reagan than Glenn? Why jump aboard a ship a good many Democratic governors, legislators asnd politicians have talked themselves into believing is starting to take on water?

The answers say something about Mondale and Glenn--and even more about Kirkland. What they say about Mondale is that, for all the political and press chatter about the dignity of his catering to "the special interest constituencies," the years he has invested in cultivating those personal and political friendships now are convertible into tangible assets. No living American politician has spent more hours in union halls than Fritz Mondale. For an organization whose motto is "Solidarity," that fact means a lot. It means Mondale knows the union leaders, understands them and will deal with them seriously. For an organization that has been shut out of the White House, not only by Reagan but by his Democratic predecessor, Jimmy Carter, that prospect means a lot--particularly against Glenn.

The rap on Glenn from the unionists is more serious than their public statements suggest. Kirkland is not going to dump openly on a man he may end up backing as the Democratic nominee if Glenn beats Mondale in the primaries. But the views he has expressed privately to associates show that the fight he will make to keep Glenn from the nomination is no casual thing. He views Glenn as a man who made almost no overtures to labor leaders in his nine years of Senate service until Glenn realized, just a few weeks ago, that the AFL-CIO endorsement would bring with it a political organization of real weight and value.

Kirkland wonders about the values of a man who seems stirred to passion by the subjects of higher education and basic research-- but not, as Kirkland sees it, by much else. He finds it hard to shake the memory that when Glenn came before the executive council in the depths of the 1982 recession and was asked about emergency jobs legislation, his response was that he'd considered supporting such a measure--if things got worse.

None of these matters may count much with rank-and-file union members, who have given Glenn overwhelming support even against union-endorsed rivals in his Ohio races. The latest Washington Post- ABC News poll shows that among Democratic-leaning union households, Mondale has only a 40-31 percent lead over Glenn.

There is a risk that labor's first early endorsement of a Democratic presidential candidate since Hubert Humphrey in 1968 could backfire. But Kirkland is not much fazed by such risks. He heard the same things two years ago when he announced plans for a massive Solidarity Day march on Washington to protest Reagan's economic policies.

He was told that it would be a bust, but the turnout of 250,000 people signaled the end of the Reagan honeymoon. The man who organized Solidarity Day for Kirkland, John Perkins, is now his top political operator, the man in charge of delivering the nomination to labor's man, Mondale.

Perkins goes into the campaign with a cadre of experienced organizers, many of them recruited from that first Solidarity Day march. He also has the best computerized list of large numbers of likely Democratic primary voters extant and a potential army of 100,000 volunteers operating out of local union headquarters in almost every community in the land.

The one vow Kirkland and Perkins have made to their union colleagues is, "Labor will not be embarrassed." That may not be just a brag.