I often am in the habit of linking to other political columns from around the web, yet I rarely find myself linking to the comments section from those pieces.  However, I thought this reader of Marc Ambinder's Atlantic posting today to be pretty spot on:

Endorsements cannot help Hillary's public perception - they only aid her fundraising, and when they come with machines, her field operation.  She started this campaign as the candidate of the establishment.  Any endorsement she lands is no more than was expected; any she fails to score is a blow.  That may be an unfair narrative, but it's the price of being the early prohibitive favorite, and of relying on your husband's standing as party standard-bearer.

Obama, on the other hand, has no stash of favors to cash in.  He doesn't control the party apparatus.  Any endorsement he lands becomes a story, and the more embedded the endorser is in the establishment, the bigger the coup.  That's why Kennedy is a huge deal.  The only reason for politicians to endorse Obama is because they believe in his candidacy.