On the subject of voter ignorance, one of the great revelations to me has been the research tending to show that the voters most likely to keep an open mind about candidates are also the most ignorant. Conversely, voters who have acquired the most political information are also the most partisan and ideological — which, on reflection, is not surprising, since it’s their partisanship that inspires them to acquire political knowledge in the first place. Hardly anyone seeks out knowledge for the (boring) purpose of making an informed voting decision.The typical undecided voter is not like Mickey Kaus in 2000, who argued with himself on his blog back and forth for ten days in a row before finally deciding to vote for Gore.
Only when you look at ignorant voters do find people who aren’t so cabined by their ideological or partisan loyalties that they will consider voting for any candidate. So, it is not only possible but probable that the voters who went for McCain have no idea what McCain actually stands for. They probably, as you suggest, use a heuristic like ”McCain is the maverick candidate and that’s what we need right now.” Little do they know that McCain actually wants to extend Bush’s failed policies.
All this was set forth in Philip Converse’s extraordinary paper "The Nature of Belief Systems in Mass Publics.” I can’t find it online, but here’s a fair summary.