Hmmm. It can be read two ways, to my thinking.
One interpretation: It is strickly a choice between Obama and McCain then the person's vote would go to McCain. That is a pretty easy interpretation, albeit possibly misleading.
However, one cannot ignore the modifying statement within the statement, i.e., "if.....McCain never selected a religious zealot as his VP". I believe that the original poster should have written, to be more clear:
"As I've said many times before, if it was just McCain versus Obama, i.e., IF McCain never selected a religious zealot as his VP, then I'd have voted for McCain."
Recall, the 'i.e.' is (in this case) a type of restatement of the preceding sentence or phrase, an explanation.
Presidential nominees never run 'by themselves'. They always have a vice-presidential running mate. The statement could well mean that the original poster was so disappointed at what appeared to be a lack of judgment by McCain in choosing Palin, that the OP's vote would go to Obama. I doubt the original poster (of the sentence in question) was assuming that each presidental candidate was running literally by himself.
Of course, I may well start a new thread to see if people agree with me. Votes will be counted.