In New York Magazine, John Heilemann has a piece on the MSM turning on McCain:
I have sat across from Chris Matthews enough times now, participating in that psychotropic ritual known as Hardball, that I thought I'd heard it all - but then the other night he uncorked a doozy that actually rendered me speechless. (No, that is not a misprint.) "Let's start with John McCain," he said to me on the air shortly after the first presidential debate between McCain and Barack Obama. "Do you think he was too troll-like tonight? You know, too much of a troll?" I laughed. "Seriously," Chris went on. "Do people really want to put up with four years of that? Of [him] sitting there, angrily, grumpily, like a codger?"
As both a media figure and a human being, Matthews is sui generis - and yet what made his comments so remarkable was how unremarkable they were. In the past several weeks, the shift of press-corps sentiment against McCain has been stark and undeniable, even among heavies such as Matthews long accused by the left of being residents of the Arizonan's amen corner. Jonathan Alter, Joe Klein, Richard Cohen, David Ignatius, Jacob Weisberg: all former McCain admirers now turned brutal critics. Equally if not more damaging, the shift has been just as pronounced, if less operatic, among straight-news reporters. Suddenly, McCain is no longer being portrayed as a straight-talking, truth-telling maverick but as a liar, a fraud, and an opportunist with acute anger-management issues.
[How John McCain Went From Maverick to Crank, October 5, 2008]
Of course, he was always like that, but they didn't feel like pointing it out as long as he was a "maverick"—a loose cannon aimed at the Republican Party.
But as Mickey Kaus points out, none of those people were ever actually McCain supporters—they're Democrats. Once McCain became the Republican nominee, opposed to Obama, the "promised prince" then McCain's days as a media idol were over.