It occurred to me while reading this Linda Douglass interview with McCain strategist Mark McKinnon how much McCain's fortunes are tied to momentum. I know this metaphor's pretty cliched, but his campaign really is like riding a bicycle: It's fine as long as it keeps moving along, but the second it stalls out, the whole thing could topple.

My thinking is that conservatives are generally going to keep their mouths shut as long as McCain has a decent shot at winning, because they know the GOP has no business even being in the game. But the second it looks like his chances are dimming, I suspect all the conservative skeptics are going to pipe up and basically finish him off.

For some reason, this was the McKinnon comment that got me thinking this way:

[I]n fact John McCain is making quite clear, as he did yesterday, on very specific issues where he does differ with the president. He respects the president and admires the president. But on fundamental issues that are very important like global warming, John McCain has a completely different position...

I know even the GOP is coming around on global warming. Still, a year ago it would have been very tough to imagine McCain as the nominee calling Bush out on global warming and not suffering serious conservative blowback as a result.

Another thing I thought was interesting from the interview: McKinnon has said he'd sit out the campaign if Obama wins the Democratic nomination. He stands by that promise and elaborates on it with Douglass, including this exchange:

Q: And it's because, what, you don't want to run negative ads against Obama?

McKinnon: Yeah.

Q: Or is there also a concern on your part that you don't want to run ads against Obama, the first African-American candidate to have this kind of a chance? Is that a factor as well?

McKinnon: I suppose that is in part, but it's more just that I like and admire the guy. I've come to a point in my life where I think character is important. I think he has great character. Again, I think he's really wrong on fundamental issues, but yeah, I just don't want to -- you know, I kind of want to put my guns down. It's just a matter of degrees, and like I said, I don't think I'm the best person to have in that slot for the campaign. So it would just be better for me to step to the sidelines.

In my piece this week, I talk about how attacks by one Democratic candidate against the other are especially damaging because they make the same attack much more credible when the GOP recycles it. But maybe it works the other way, too. Maybe being able to say that McCain's top media advisor quit rather than run negative ads against him would give Obama some immunity from the attacks McCain sends his way. You can imagine him saying, in the face of a hard-hitting ad, something like, "McCain's own media advisor quit rather than run an ad like this because he thought I had great character. So that shows you how misguided this is..."

--Noam Scheiber