Last summer, Mark McKinnon, the former chief media adviser to George W. Bush, and now a top aide to John McCain’s presidential campaign, admitted that he liked Barack Obama so much, he didn’t want to work on the campaign running against Obama and he wouldn’t create negative ads against the Illinois senator if he became the Democratic nominee.

Of course, that was easy to say at the time. A year ago, McCain’s campaign was in deep trouble, and Obama still seemed like a relative long-shot. Given that we’re talking about a top-tier politico who was Bush’s top media guy, I assumed he’d come up with some excuse to change his mind.

I assumed wrong. Chris Cillizza reports that McKinnon actually meant it.

Mark McKinnon, the lead media consultant for Sen. John McCain’s (Ariz.) presidential bid, is stepping down from that role — making good on a pledge he made last year not to work against Sen. Barack Obama (Ill.) in the fall campaign.

McKinnon confirmed his decision to The Fix this afternoon. “I’ll be transitioning, shifting position from linebacker to head cheerleader,” said the always-colorful McKinnon. He added that he would continued to be a “friend and fan” to the campaign.

McKinnon is not just some low-level staffer — he’s been responsible for shaping McCain’s media strategy. He’s also a trusted member of McCain’s inner circle and Cillizza added that “many within the campaign acknowledged that losing him would be a heavy blow to the effort.”

And now he’s leaving the playing field to watch from the sidelines, because he just likes Obama that much.

To get a sense of his perspective, consider that McKinnon told NPR last year, “I would simply be uncomfortable being in a campaign that would be inevitably attacking Barack Obama. I think it would be uncomfortable for me, and I think it would be bad for the McCain campaign.”

I guess McCain deciding to stay positive and not attack Obama wasn’t an option.

I have to say, this is just not normal. I can’t think of a comparable example of a major candidate’s key advisor stepping down because he or she liked the other party’s candidate so much.

I suppose this leads to two questions:

1. Why would McCain hire a media advisor who was prepared to quit if Obama became the Democratic nominee?

2. Why would McKinnon go to work for McCain if he were prepared to quit if Obama became the Democratic nominee?

I suspect the answer to both is the same: they assumed Clinton would get the nod and this wouldn’t be an issue.

I guess kudos to McKinnon are in order. I never really thought he’d go through with this, giving up a high-profile, lucrative job for the Republican presidential nominee. The follow-through here is pretty impressive.