Since he was about to mention “liberal democrat” after saying “conservative republican”, it seems an understandable mistake that a HUMAN BEING would make.
Matthew 12:34
You brood of vipers, how can you who are evil say anything good? For out of the overflow of the heart the mouth speaks.
In other words, people can’t help but reveal what is truly in their hearts by the words they speak.
All of that would be true if McCain were a real Conservative. He's not, he's lying now to try to make us think he is, and thus, his misspeak is more than a mistake.
Context.