I think Josh Marshall is on to something.

You have to step back for a moment to grasp the sheer preposterousness of the suggestion that Mitt Romney’s time at Bain Capital might really be off-limits in the presidential campaign.

Think about it. Romney’s presidential campaign, in the sense of his case for himself, is almost entirely based on his experience at Bain Capital. You hear extremely little from Romney about his tenure as Governor of Massachusetts. Rather, his case for himself is that he’s a seasoned manager and knows the private sector — both of which he has a pretty strong case for. But his entire private sector career was at Bain Capital. So how exactly can that possibly be off-limits. Romney hasn’t just raised the issue; it’s his entire campaign.

Which explains why the Obama campaign is ignoring the blips from Cory Booker and Ed Rendell and going all in on the Bain Capital story as the metaphor for a Mitt Romney presidency. There is nothing else in his background that will appeal to the base (and when I say “base,” I’m talking about those baying-at-the-moon loons who are writing the party platform in Iowa).

That also explains why the Romney surrogates like John Sununu are all wound up about it, calling the ads an “attack on the American way of life” (which makes you think that Mr. Sununu has the entire collection of Robber Baron bobbleheads in his study). But even he admits that Romney’s tenure at Bain is “fair game.” (Wait; did John Sununu have a Cory Booker moment? Stop the presses.)

Now that we know that this is a sore spot — indeed, the only spot — for the Romney campaign, expect to see a lot more about Bain from here on out.