At least according to Dan Riehl at Riehl world view who recalls an old Sullivan Post per election concerning Barack Obama:
On October 27, 2008, Andrew Sullivan posted: The Top Ten Reasons Conservatives Should Vote For Obama. Given all this playing out today, I thought I’d go back and have a look.
It’s high comedy but let me explain something. It’s my opinion Sullivan’s turn on Bush had everything to do with Bush’s position on Gay Marriage, it was after that point where Sullivan really started changing his tune and it was the (likely correct) belief that Obama was paying lip service to actual marriage during the campaign that made the difference in the other direction.
But as Glenn Reynolds points out the rubes are self identifying, but never fear. As soon a there is an actual Republican challenger to Obama all of our friends on the left who are beating their breasts today will support him, Mendoza line or no.
Update: Stacy Links and comments seem to agree with my Sullivan assessment, Oh and BTW the Mendoza line is a baseball term referring to hitting .200.
Da Techguy's (no longer backup) Blog 
[…] asking questions” — demanding to see Obama’s long-form birth certificate?UPDATE: Linked by Da Tech Guy, who points out that Sully’s turn against Bush was manifestly motivated by (ahem) domestic […]
This is absolutely true. I had a brief email exchange with Sully back when the whole DOMA/gay marriage thing was playing out and, to say the least, Sully’s responses were unnecessarily shrill and incoherent. He’s simply not approaching the topic with any sense of detachment.
You are entirely correct about when Sullivan started to turn on Bush. I used to read his blog regularly, and even contributed during his fund-drive, so I could see the change in real-time. When Bush came out for a constitutional amendment against gay marriage (which I also opposed), Sullivan began to air more criticism from the Left of the President. Over the next several months, the links to folks like Josh Marshall increased as opposed to NRO, Instapundit, etc. The rest is a sad history of the descent into conspiracy-nuttiness, ad hominem attacks, and parroting of far-left talking points.
I decided that a blog should be created on the hype and hysteria generated by much of the news today… and what would some of it look like if taken to its logical conclusion. I plan on starting some other headlines taking some of the current news taken to its most hyperbolic.
If the world was consistent this would be the headlines today…