The field is crowded, but here is one way to extend invitations, based on job experience.

Governors have unique qualifications because they have to work with the two branches of legislators and state court decisions.  They're mini-presidents.  The other candidates have their own set of skills.  If they're in the government, they're legislators.  If they've been in the private sector, they don't have an understanding of the slow-grinding wheels of government, but they can offer other things.

So let's divide the debate invitees as follows:

1. Governors in one debate, which include Bush, Perry, Pataki, Christie, Huckabee, and Jindal; if these announce: Walker, Kasich, Ehrlich, and Gilmore.
2. Legislators and others in the other debate: Senators: Paul, Rubio, Graham, and Santorum; CEOs: Fiorina and Trump; other than CEOs and legislators: Carson, Everson, and Fellure.

A minimum polling level – say, one percent – could be established for the second group.