The AFL-CIO's decision to try, at least, to endorse a Democratic presidential candidate in December 1983 is seen, correctly, as an attempt to restore organized labor's power in national politics. But it is important, for the labor movement and for others, to understand just what labor's power used to be if we are to have a sense of how a more powerful labor movement will exert power in the future.

According to the conventional wisdom, labor's power came from its electoral clout. It contributed thousands of dollars to candidates, it swung hundreds of thousands of votes by its endorsements, it contributed tens of thousands of campaign workers to the Democrats. All true, to a point. But when you look back on it, the size of labor's contributions wasn't all that great in the 1950s and 1960s, a time when Democrats were able to win most elections without the expensive media campaigns that are necessary today.

There was a time when some union endorsements swung votes, but not many of them are more recent than the 1968 election. Even then, there were already defections, to Richard Nixon and George Wallace, from Hubert Humphrey, despite his enthusiastic support from union leaders. More recently, in many elections the union vote is only five or 10 points more Democratic than the national average.

As for primaries, in heavily unionized Pennsylvania, labor wasn't able to beat Jimmy Carter in the decisive primary of 1976. What about all those campaign workers? With the exception of the NEA (teachers) and CWA (telephone operators), few unions can muster many of the articulate, organizationally adept volunteers who are useful today.

The fact is that union members are a much smaller part of the non-farm work force today (24 percent) than they were in 1946. And few members see their unions, as many did 40 years ago, as the only reliable suppliers of information.

For most of the last 20 years, organized labor has been a major political force not because of its prowess in campaigns but because of its skill as a lobby. In the 1950s, 1960s, and most of the 1970s, labor was the one major Democratic-oriented lobbying force on Capitol Hill. It was, moreover, a broad- minded lobby, not concerned only for its own narrow interests: labor was a major force behind the civil rights laws of the 1960s, for example.

On issue after issue, almost every non-southern Democrat, plus blocs of northeastern Republicans and southern Democrats of about 20 House members each, followed organized labor on almost every roll call vote. Labor set and enforced the agenda for the Democratic Party. Its campaign contributions helped, but most important was the fact that its skillful leaders -- men like Andrew Biemiller and Al Barkan -- were the people who did the work.

All that ended, with unusual suddenness, in 1977. Carter had just entered office, the Democrats had big majorities, and labor assumed it could get just about everything it wanted. First on its priority list was a bit of old business: the common situs picketing bill, which had passed the previous Congress only to be vetoed by President Ford. Its purpose was to allow the building trades -- which account for a large share of the AFL-CIO -- to picket an entire building site whenever one union wanted to picket one subcontractor.

Unexpectedly, the bill lost in the House. The building trades, after all, were very few voters' idea of a deserving minority. And the newer Democratic members were more interested in pleasing their constituents and following their own lines of thought than they were in following labor's lead.

From there, things fell apart for the labor lobby. Many non-southern Democrats deserted tradition and supported deregulation of oil and natural gas. The labor law reform bill -- a much more attractive cause than common situs -- failed. In 1978, the House started out trying to raise taxes on the rich and ended up cutting the capital gains tax. Biemiller and Barkan retired; their successors, though able, weren't able to restore labor's clout as a lobby.

Now Lane Kirkland is trying to make labor into what it hasn't been for a very long time: a force in the Democratic presidential selection process.

It's a risky business. The AFL-CIO may not be able to summon up the two-thirds consensus needed for endorsement, or -- given many voters' hostility to union leaders -- the labor endorsement may turn out to be a liability rather than an asset. Historians may consider it odd that the labor movement has turned from emphasis on lobbying to emphasis on electoral politics even as its membership, as a percentage of population, continues on a long decline. This is not to say that labor is not an important lobby; it is, but it hasn't been the de facto leader of the Democratic Caucus for six years now. And it doesn't mean that labor might not be successful in the 1984 presidential race; who knows? But it does mean that labor, even with its thoughtful leadership, is having trouble finding a mission and a comfortable role in American politics.tion: Stamp By George Rebh