THE DECISION of the AFL-CIO Executive Council to put the question of endorsing a presidential candidate before the organization's national convention in October is being analyzed in tactical terms. The consensus is that the change in date helps Walter Mondale and hurts his Democratic rivals. Mr. Mondale is expected to win the National Education Association's endorsement easily on Sept. 30; a victory, with the required two-thirds vote, at the AFL- CIO convention a few days later would give him precious momentum and the support of thousands of political activists. But there are also risks. Union leaders aren't popular with most voters, and Mr. Mondale is criticized for promising too many "special interest" groups what they want. And if the AFL- CIO's candidate loses the nomination, many will say labor has lost its clout.

It will be obvious soon who will get the labor endorsement, and what effect it will have on the Democratic contest. More interesting in the long run is the question, what do labor's leaders want? First, they want to increase labor's clout. And they want to get the kind of president they'd like. Most union leaders haven't been comfortable with any Democratic nominee since 1968, and this time they want to make sure that any Democrat who is nominated genuinely shares their views. That's why there is little support among these leaders for John Glenn; he may have a good AFL-CIO voting record, but union leaders are not confident that he'll do what they consider the right thing once in office.

There is no sign that labor leaders are extracting specific quid pro quos from Mr. Mondale or other Democrats. But to understand what they are likely to seek if a union-endorsed Democrat wins, it is useful to understand that many union leaders today believe their movement is under siege. The big industrial unions have lost hundreds of thousands of members over the past five years. Government employees and teachers were the biggest sources of new union members in the 1970s, but there's not likely to be a vast increase in public employment soon. In industries traditionally resistant to unionism (such as textiles) and in those traditionally unionized as well (such as meatpacking), employers have taken the offensive against unions with great success. Union leaders believe that many employers, large and small, are trying to put the union movement out of business altogether. If that seems implausible, consider: in 1954 union members accounted for 35 percent of the non-farm labor force; now that figure is below 25 percent, and falling.

So a first priority for the AFL-CIO is almost certain to be measures that strengthen unions as collective bargaining agents--something like the labor law reform bill rejected in 1978. It is also likely to champion the parochial interests of the building trades by backing public jobs programs and opposing changes in the Davis-Bacon Act. Yet the AFL- CIO has a tradition of representing not only the narrow interests of member unions, but also a broader public interest generously defined--most notably on civil rights. A key question for American politics is whether the union movement can keep this tradition a lively one even as it goes about the more practical business of political organization and seeking to preserve a movement that feels its existence is threatened.