There's only two simple sides to it: Either He was the reason, or he wasn't, and I personally don't think its either. He would have endorsed who ever was on the ticket, since he was a Democrat. The US citizenry needed something different what the republican government had been doing, and was offering to do if they gotten elected. And they got that in Obama.
Regardless of who actually got the Democratic nomination, the Republicans had no hope of getting into office. Bush screwed the pooch constantly for 8 years and the country were sick of it. The landslide victory for the Presidential electoral college, and a majority in Congress should stand testament to that.
and before you all start going "Obama isn't keeping his promises" and all that jazz, let me remind you of a very simple fact: Getting elected and Doing the job are two COMPLETELY different things