Didn't he also endorse Hillary over Obama in 2008?
Not only did he endorse Hillary, he was one of her very early backers.
At the time, it was too soon in his gubernatorial career for him to run, and he probably looked at the rest of the field and noted it consisted of:
A Senator in the middle of his first term
A one term senator who was on the 2004 losing ticket
Two liberal loons (Kucinich and Gravel)
Two candidates who he may have liked but who he knew had no chance at the nomination (Biden and Dodd)
So, that left Richardson and Hillary, so he endorsed the better GE candidate (Hillary). It wasn't a full-throated endorsement, just supporting the candidate who obviously looked like "the best of a bad field" at that early point.