The rise of Mike Huckabee in polling is leading some to suggest the possibility that Rudy Giuliani might choose him as a running mate - or even, if Huckabee continues to rise, that Huckabee might choose Giuliani.
For either scenario to arise, the candidate would have to ignore one of the iron rules of choosing a running mate: outsiders always choose insiders.
First, to definitions. An outsider is nearly always a Governor, though being Mayor of a city with a population greater than most states is, plainly, equivalent experience. Insiders are usually Senators - Representatives, cabinet Secretaries, and Vice-Presidents also count. I exclude serving Presidents from the analysis, as they really each sui generis.
Every Governor to be nominated for President in the last 50 years has chosen a Senator as running mate except Ronald Reagan who chose a former Representative and cabinet Secretary. It is true that insiders sometimes choose other insiders: George H W Bush and John F Kennedy did, and both won. But every other ticket made up of two insiders - Kerry-Edwards; Gore-Lieberman; Dole-Kemp; and Mondale-Ferraro - has lost.
While it seems possible that Hillary Clinton would choose as her running mate a serving or former Senator - Obama, Bayh, even Edwards again - it is more likely she would choose someone with gubernatorial experience, and probably with a regional base. Mark Warner or Bill Richardson would be the most obvious choices. Of the Senators, Evan Bayh, who is also a former governor, would be the strongest, but probably could not deliver his state.
If Giuliani or Romney wins the GOP nomination, the running mate will be a Washington insider of some description. Huckabee will not make the shortlist. Sam Brownback, Mike Pence, Tom Coburn, John McCain, Fred Thompson, Newt Gingrich and even Dick Cheney would be worth considering. Of those, McCain does little to reassure southern conservatives, who would be likely to have doubts about either Giuliani or Romney. He would balance Romney's perceived lack of experience in areas of national security. This is not, however, an area generally held against Giuliani.