Good point and I realize that not all of the members are liberals however even if 1/2 of the members support Obama that is huge for him.
My point is this - Obama will be the Dem nominee. It is almost impossible for Hillary! to catch him with pledged delegates. The super-delegates will break heavily for Obama if he is ahead in pledged delegates and overall vote total. She’s toast - and she knows it...
The SEIU endorsement may indeed put Obama over the top; they have some of the best field organizers in labor movement.
Of the SEIU members who vote in Democrat primaries, Obama may get most of the votes; but the real strenght of the endorsement is the get out the vote for Obama that will occur.