Rare is the deed that confounds the cynics and at the same time makes sense. The improbable proposition of this column is that the AFL-CIO (a k a Big Labor) may well have demonstrated both its sincerity and its shrewdness in deciding last week to postpone any endorsement in the 1988 presidential race.

Union leaders regularly rank even lower than journalists in polls of public trust. It was no great feat for critics of Lane Kirkland and his brethren in the federation leadership to sell the notion that the 1984 endorsement of Walter Mondale was dictated by the ''union bosses.''

Mondale had been labor's loyal ally from his first days in Minnesota politics. It would have been rank ingratitude for the AFL-CIO to deny him its support. Nonetheless, he never escaped the ''special interest candidate'' tag, and labor never persuaded most people there was anything very democratic about its endorsement vote.

Thus, when Kirkland announced early this year that the AFL-CIO would canvass its members' sentiments and endorse a 1988 hopeful ''if there is a consensus,'' the statement was greeted with knowing nods and winks. The smart guys figured Kirkland and the leaders of the six or eight biggest and most politically active affiliated unions would sit down and decide who they wanted -- and that would be it.

Wrong. Instead, the unions flooded their local leaders and members with biographies, voting records and videotapes of all the candidates (including the Republicans) and then polled them repeatedly on their preferences. The answer came back that the membership was all over the lot, with only George Bush and Jesse Jackson breaking into double digits.

''There is no consensus within our ranks at the local or national level,'' Kirkland announced. No consensus, no endorsement. As simple as that.

Except it isn't. The deferral of the AFL-CIO endorsement changes both the perception and the reality of the Democratic contest. The candidates' first test now is to win not the endorsement of interest groups, but the support of voters in the early caucuses and primaries. That change was one of the earliest and most important objectives announced by Democratic National Committee Chairman Paul G. Kirk when he took over after the 1984 Reagan landslide.

When Kirk first asked labor, teacher and other interest groups to forgo or delay their endorsements in 1988, they told him to mind his own business. He has. But circumstances have yielded the result he wished, and he is smart enough not to gloat.

Also not gloating but privately rejoicing are the candidates who were unlikely to get the AFL-CIO nod: Bruce Babbitt, Jesse Jackson, Albert Gore Jr. ''It levels the playing field,'' said Robert J. Keefe, a longtime Democratic operative with close ties to labor. It also frees candidates to exploit their alliances with particular unions in particular places, Keefe noted. That may help Jackson in some urban areas. It will aid Michael Dukakis, Paul Simon and Joe Biden in and around their home states of Massachusetts, Illinois and Delaware.

But the biggest early beneficiary could be Richard Gephardt, who has worked tirelessly to win unionists' support in Iowa, site of the first caucuses. He now knows those labor people are free, as individuals, to work for him and become his delegates.

Under the recommended rules, no union is supposed to make an official candidate endorsement until the AFL-CIO does, but unions can help union members become delegates. It's a thin line, and Paul Jensen, who was Mondale's liaison with labor in 1984, thinks the unions will have to blur it further if they are going to be at all effective.

''No candidate is going to slate labor delegates unless the union is working for him,'' Jensen said. ''Everyone knows that.'' Jensen, who is now counseling Dukakis, is also of the view that Kirkland and Co. will have to discover the missing consensus and make an endorsement by mid-January at the latest or risk becoming bit players in the 1988 nomination drama.

But that is not likely. Indications are that Kirkland sees mid-February, right after Iowa and New Hampshire have voted, as the best time for labor to deliver its blessing, its dollars and its field organization to someone who has survived the early tests.

''My guess,'' said Keefe, ''is that Iowa and New Hampshire will winnow the field and make one or two people the favorites for Super Tuesday,'' March 8, when simultaneous primaries and caucuses are held in 20 states, 14 of them in the South.

''Labor may be in a position to cement a base under the guy that's flying high after New Hampshire,'' Keefe said. ''If labor comes in with an endorsement the day after New Hampshire, that could just about put him over.''

It would be quite something if Kirkland, by playing straight, turns out to be that shrewd. What would the cynics say then?