THE ANTICIPATION preceding the AFL-CIO's endorsement of Al Gore's presidential candidacy is a tribute to the evolution of the labor movement. As a force in the workplace, the unions have long been declining. But as a force in politics, they have by many accounts grown stronger. Both Mr. Gore and his rival in the Democratic primaries, Bill Bradley, lobbied hard to win labor's endorsement. Mr. Gore thought the contest so important that he called in President Clinton's help to pull the unions to his side.

It is easy to see why the candidates are anxious. There are 160,000 union members in Iowa and 50,000 in New Hampshire; and union members are likelier to vote than the average American. In California, which may well decide the outcome of the Gore-Bradley contest, the unions proved their electoral muscle last year by defeating a ballot initiative designed to curtail their political expenditures. In the 1996 and 1998 elections, exit polls showed that 23 percent of voters came from union households, up from 19 percent in 1992.

For Mr. Gore, therefore, labor's endorsement provides a welcome boost to a flagging campaign. For labor, too, there are potential benefits. If the endorsement is seen to help Mr. Gore win the primaries, the unions' image as kingmakers will have been strengthened, along with their influence over the nominee's policies. Moreover, if union organizers tip the race decisively in Mr. Gore's favor, the chances of the Democrats' keeping the White House may go up a bit. Decisive primary victories conserve the winner's cash and energy, leaving him stronger in the general election.

However, a Gore primary victory is by no means certain. Mr. Bradley's campaign is gaining momentum: Money flows into his war chest, and his poll numbers are good. Even though union families loom large in Democratic primaries, the AFL-CIO can only influence, not determine, which candidate its members vote for -- and Mr. Bradley's policies are as pro-labor as Mr. Gore's. If the unions turn out to have endorsed the loser, their vaunted political clout will suddenly seem empty. That would hurt the labor movement deeply, given that its reputation hangs increasingly on its apparent political muscle.