"John McCain has become the de facto running mate of George W. Bush"

This quote suggests that John McCain has become an informal partner to George W. Bush in his governmental project. It indicates that McCain has actually been a strong supporter of Bush and has been actively associated with his campaign. This could imply that McCain has actually been supplying recommendations and guidance to Bush, or that he has been actively campaigning for him. It might also suggest that McCain has been a vocal advocate for Bush's policies and has been helping to spread his message. In any case, it is clear that McCain has ended up being a vital part of Bush's project and has been a major consider his success.