Post-mortems abound in Washington on President Clinton's failure to get "fast-track" trade-negotiating authority through Congress. One reason, analysts suggest, is because he waited too long to push for it, and then didn't have formal legislation ready until this fall. That gave time for organized labor, fast-track's biggest opponent, to line up commitments among Democrats against the measure. Also, the business community, which was absorbed in other lobbying fights, entered the fray too late to be effective. Fast-track supporters pledge to make another effort to pass the legislation when Congress returns in January. But passage will be even more difficult than it was this year because 1998 is a congressional election year.