For unions, this year marks the best chance in 15 years to win favorable changes in the nation's labor laws. Unfortunately for the AFL-CIO, however, the best may not be good enough. The top item on labor's agenda is legislation prohibiting companies from hiring permanent replacements for striking workers. The House, which passed the measure easily in 1991, is expected to approve it again sometime in the next few weeks. But in the Senate, supporters will need 60 votes to break an expected filibuster. Last year, they fell three votes short, and they may have trouble coming that close again. In 1992, Bob Packwood of Oregon was one of just five Republican senators voting to end debate. But because unions abandoned him in his tight reelection race last year, he may not support the bill now. In addition, some conservative Democrats voted for cloture confident that President Bush stood ready to veto the striker-replacement bill. Bill Clinton could probably pull the measure through the Senate with an all-out effort. But while he's deeply in organized labor's debt, that's unlikely to happen. Deficit reduction and health-care reform remain much higher priorities for the Administration, and the President can only afford a limited investment in the labor measure.