The move to the political center once the nomination has been secured is as strong a force as the gravitational pull. You can fight it but in the end succumbing to it not an option. That said it's also a smart move.
The candidates are chosen largely by political activists. Even more so in the Democrat party where super delegates are in play. Party activists are more idealistic and cast their gaze on a more pure idealogy that reflects the party's platform. Things like a "Department of Peace" and "No War for Oil" are more than slogans they are deeply held beliefs among activists. If you want those votes then you need to talk like one of them.
Where once only angry Americans foolishly clinging to guns and bibles existed in fly over country, you suddenly become a hunter and love the 2nd Amendment. Where once you railed against the war in Iraq you suddenly see the need to seek counsel from the Generals. Where once you seemed angry at an America that is fundamentally unfair and unjust...well...what's that on your lapel?
Thankfully there is enough freedom in this country left that people don't engage themselves in the political process until there are actual nominations and now the great unwashed independents and undecideds need to be convinced. At this point of the game it's party platform be damned. McCain has done it also as he has become every so suddenly interested in global warming.
Where it becomes a problem for Obama is that it turns his greatest strength into a weakness. No, it's not integrity so listen up. It's his position as the outsider. All he has to do is utter the word "change" with no details and it becomes a code word for "one of us." Unfortunately for Obama, When the pull to the center happens and you don't have a 30 year track record to draw upon, it makes you look like an opportunistic flip-flopper. This makes him vulnerable to attack but doesn't necessarily doom him.
I have to confess that I'd just love to see candidates run on their record and stay principled. If our choices only fell on two principled people we'd have to pick one and who knows...maybe we might actually respect one of them.
Oh, one more thing. Bob Barr is polling in double-digits in a few states. Just sayin'