The issue that won’t go away, McCain leading Romney or not, is McCain as someone who knows how to run things:

No offense to motivational speakers, but real-world management skills are useful too. That’s a McCain weak point.

Says Collin Levy in the subscription-only Wall St. Journal Political Diary. 

The truth is, Mitt Romney’s management skills are unquestioned by those who dealt with him at Bain Capital and the Salt Lake City Olympics, whereas Mr. McCain’s reputation for staff work at the Commerce Committee was less than stellar — in fact, it was a source of consistent and vocal complaint by those who did business with the committee.

It’s the economy, stupid, it’s the war vs. Islamo-fascism, it’s judges, it’s tax-cuts, it’s lots of things.  But the man in charge better know how to run the show.