McCain has an issue with gaining support from the Religious Right… the hard core base of the Republican Party. A Baptist Minister for a VP helps to solve that problem. Also, with Huckabee as a Fair Tax supporter, it helps to reduce the “vote bleed” from any Libertarian candidate. Not a huge factor, but as we have seen in several recent elections even a small 3rd party vote can play havoc with the national results.
As for Richardson with Hillary, that broadens her appeal. She has “roots” in Arkansas and New York. Richardson in the Southwest. Richardson polls very well among Latino voters, Hillary polls well with Caucasians. And it is not like the Black vote is going to vote for McCain… the Democrats can again take that for granted.
So, as always, it comes down to a fight for the middle. McCain is strong among moderates of BOTH parties, but lacks the ability to energize the Republican Base. Hillary, by her own actions, is seen as a New England elitist and needs that diversity from another part of the nation to help push her over the top. Richardson also has strong support among the typical McCain voter, and would help Hillary actually take some “safe” McCain votes away.
Clinton/Obama will NOT happen. Hillary destroys her enemies, she does not make them into Vice Presidents. Guiliani has not shown anything that would make him be a positive choice for VP. His BEST showing was 4th. And McCain will NOT choose Mitt for a VP. Too much bad blood there. That leaves Huckabee… to unite the party (McCain’s 30 something percent and Huckabee’s teens for a functional majority)