All this talk of potential new VP candidates (for both parties now) made me want to look back for some sort of perspective on the issue. What follows is an analysis of the 22 Vice Presidential nominees (or, in the case of Ford and Rockefeller, appointments) since the election of 1952, not counting sitting VPs. I'm going to try to avoid stylistic and personality-based arguments in favor of more quantifiable attributes.
Of these 22 VP nominees, the vast plurality
(13) were Senators:
Nixon '52, Sparkman '52, Kefauver '56, Johnson '60, Lodge '60, Humphrey '64, Muskie '68, Mondale '76, Dole '76, Quayle '88, Bentsen '88, Gore '92, Lieberman, '00
The next largest group are former Administration or Cabinet officials (5):
Lodge '60 (UN Amb., Amb. to Vietnam), Shriver '72 (Asst. to LBJ, Amb. to France), Bush '80 (UN Amb., CIA Director), Kemp '96 (HUD Sec.), Cheney '00 (Defense Sec.)
Only (3) were Governors or ex-Governors:
Agnew '68, Muskie '68 (also Senator), Rockefeller '74 (appointed)
And while 14 of the above were at one point Representatives, the vast majority had some other, more important experience (such as Senator or CIA director), and only (3) sitting Representatives were nominated (or, in Ford's case, appointed):
Miller '64 (also RNC Chair), Ford '73 (appointed), Ferraro '84
So let's apply this to the GOP in 2004. Firstly, in the few cases where a sitting VP was replaced, by necessity or choice, the choice was usually a safe one (and all revolved around Ford). When Agnew resigned, Nixon appointed Ford, minority leader in the House. When Ford ascended to the top of the ticket, he chose Rockefeller, a national figure and party insider. And when he decided to replace Rockefeller, he did so with Dole, another safe Senator.
So if Cheney goes - by necessity or by choice - one would expect Bush to again go the safe route. In this case choices like Frist and Ridge seem more likely than choices like Rice or Giuliani. The positives of choosing a more dynamic figure like the latter two also include many more negatives - not the least of which is pissing off his base even more. The safe route seems the likely route here.
Now let's look at a potential Kerry candidacy. There has only been one Senator-Senator ticket in the past 50 years: Kennedy-Johnson '68. There have been only (2) Senator-Representative (or Representative-Senator) tickets: Goldwater-Miller '64, and Dole-Kemp '96. We all know Governors and Vice Presidents have been almost exclusively the presidential winners over the last half century. Only a handful of Senators have even secured their party's nominations over that time period: Kennedy, Goldwater, McGovern, and Dole.
So Kerry could try to emulate the last Senator to become president (the one with his initials), and forge a Senator-Senator ticket which unifies the party geographically and ideologically. (BTW, the last Senator-Senator ticket before JFK-LBJ was James G. Blaine-John A. Logan for the GOP... in 1884! And the last winning Sen-Sen ticket was Pierce-King in 1852.)
The problem is, while there are plenty of options to unify the party geographically (Bayh, Breaux, Graham, Edwards spring to mind), Kerry himself has done a very good job of unifying the party ideologically already. And if he is going to try to do so, he'd be better off picking someone who could convince the Dean/Nader wing of the party than picking a DLCer. If he knows what's good for him he'll avoid choices like Bayh or Breaux (or Warner outside the Senate). The choice would probably be Edwards or no one if he goes this route. Two senators who are on the same page ideologically run the risk of being ultra-monotonous, but Kerry and Edwards may have enough appeal to disparate groups to pull it off.
Kerry has less of a reason than Bush to play it safe with his choice. He can afford to do something dramatic, and race and gender are the obvious ways in which to do that. He could also choose someone as a complete surprise, someone from outside government even (with ultra-strong credentials obviously). But I'd bet that if it's not Edwards, it won't be a Senator. And if it's not a Senator, it won't be a white man.