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.
You are correct! I have watched the GOTV machine they have - it is tough to beat...
WI however is filled with other kinds of unions - steelworkers, etc. - the kind that like the Clintons.