I think Obama thinks that someone who shouldn’t have leaked to the McCain campaign information that should not have been publicly available so that the campaign could use it to political advantage. I think that he has this impression based on information in Palin’s speeches that was not publicly available. One reason it was not publicly available may be that all they have is a string of allegations that may or may not be founded.
I believe that the firing of the US Attorneys that has come under fire were allegedly fired because they refused to pursue allegations into voting fraud that they felt lacked basis in evidence. In other words, all they had were accusations, and nothing to back them up to make the accusations credible.
My feeling is that they wanted these allegations publicly investigations in order to intimidate potential voters and make later accusations seem more credible. They want to cast doubt and fear. Fear so fewer people will register and vote against them, and doubt about the legitimacy of the new administration so that making changes will be more difficult, as will be finding out what has gone on during this administration that ought not to have.