Edwards and labor

From Steven Greenhouse:

It often appears that the thing that will most help Mr. Edwards secure more union endorsements is not for him to march on a union picket line for the umpteenth time, but for him to get a 5 or 10 percentage point bump in nationwide polls.

I think that's probably correct, but it's indicative of a labor trend I find bizarre. If labor both conceives of itself as (and truly wants to be) a momentous force in American politics, then they should be demanding the sort of fealty from politicians that Edwards has been offering, and refusing to endorse candidates who are insufficiently willing to say publicly the things Edwards has been saying publicly. If certain unions want to endorse John Edwards--as has been pretty clearly telegraphed--but they waver because they don't believe they're formidable enough to withstand the blowback if Hillary Clinton wins, then how strong can they possibly be?