One could, theoretically, take donations from donors and not be beholden to them. I know it's easy to be cynical- and probably prudent- but Obama seems to be able to convince an awfully big contingent that he's sincere.
It takes gobs of cash to run a campaign. Is there one candidate who is taking federal funds and eschewing the more lucrative private financing? The federal funds are just not enough to cover all the expenses these days- especially if you want to become a frontrunner.