Last night, CNN aired a terrific segment that spent a solid eight minutes taking apart one of the core arguments at the heart of Mitt Romney’s case for the presidency. It looked at Romney’s ongoing claim that the “net” jobs lost on Obama’s watch prove his policies failed — even though that metric factors in huge job losses at the start of his term, before those policies took effect. And it also looked at the Romney camp’s insistence on a different standard for itself — that his record should be judged by the job gains later in his term.

That’s good, but it gets better.

On the segment, a top Romney adviser inadvertently admitted, as clearly as you could want, that the whole “net” job loss calculation is thoroughly bogus, undercutting Romney’s whole case.

Watch the whole thing (the video was clipped by the pro-Obama American Bridge), but the key moment comes at around the 3:10 mark:

Kerry Healey, the former lieutenant governor of Massachusetts and a member of Romney’s inner circle, was asked whether Romney’s aides have made a mistake in the use of their double standard. Healey then criticized the Obama administration for attacking Romney’s record as 47th out of 50 states in job creation by saying:

“The Obama administration has been trying to characterize the entire Romney administration with one number, which they keep saying again and again: 47th out of 50th in job growth. But in truth, really it’s a progression. “

Healey then argued at length that Romney had slowly brought the state up to 30th in job creation, and pointed out that Romney had brought the unemployment rate down in the state, unlike Obama. But when pressed again on whether Romney should be held accountable with a metric that includes early job losses, she said:

“Do you embrace this notion of averaging? Should we average the four years? Does that make any sense? Does that tell the voters anything? What the voters want to know is, What direction are you moving?
“Using one number, this odd average of four years, to come up with 47th in the nation, doesn’t really show what happened, which is a progression towards ... full employment in Massachusetts.”

This “notion of averaging,” this use of “one number,” is precisely — precisely — the metric that the Romney camp uses to make its main case against Obama, i.e., that the “net” overall job loss on his watch shows that he destroyed jobs.

Now, if the Romney camp wants to start using just the unemployment rates to compare the two records, that’s one thing. And no one is saying that Obama’s policies haven’t underperformed. But at this point, the metric that until now has formed the foundation of Romney’s main argument against Obama has been thoroughly discredited.

Between this CNN segment and Michael Shear’s terrific New York Times piece yesterday, it looks as if the big news orgs are starting to get serious about getting to the bottom of this. One more, and we’ll have a trend!