The big problem for Barack Obama in making this argument is he hasn't exactly spoken truth to power since being in the Senate. Also, he's run, at least up until now, a very conventional campaign and has played down or dodged all together tough Senate votes on Iran and the statement condemning Moveon among others. He also speaks with less clarity on issues like Iraq and FISA than do some of his other rivals for the nomination like Bill Richardson and Chris Dodd. So while I count myself among those who hoped he would bring a new paradigm to presidential politics and run a different kind of campaign, at least until now he's been entirely conventional with the possible exception of his fundraising, where he's continued to do some of the things the Dean campaign pioneered like hosting small dollar events.
Lastly, I think his campaign's responses and criticisms of his rivals at times has seemed off-key with their candidates broader message of changing the tone. This in turn allows the Hillary people to repeat that he's "abandoned the politics of hope" and makes you wonder if Obama was genuinely ever a different kind of politician or if he's just a gifted wordsmith who came up with some nice sounding lines for his 2004 convention speech and was hoping he could simply ride that all the way to the White House.