I don't think that there would be that much support for Tim Cahill, unless he has billions to spend on a campaign (maybe Mike Bloomberg can help him).
So, Cuomo-Castor or McDonnell-Walker?
I just don't see either of these candidates being that exciting. Kathy Castor would probably be able to rally the Democratic base. I think Andrew would need to come out and be the progressive candidate - supporting marriage equality, the environment, the working poor, and of course, a public health insurance option (since I do not think that the support now in 2009).
McDonnell-Walker would be a generic Republican ticket.
Kathy Castor's Florida would swing her way. There are a lot of old New Yorkers in the Sunshine State too, and they will have fond memories of Andrew's father, Mario, who was Governor in the 1980s.
The Northeast would be Cuomo-Castor Country. There are a lot of urban ethnics up there and a lot of Italian-Americans and Mario would be the first Italian-American President.
The "Obama South" will recede back to red state status, and this will be the first election where Florida begins a century-long transition into a blue state. (Keep in mind that Castor is from the Tampa area, which is Swing City in FL. Her presence on the ticket will cause FL to be more blue than usual).
The McCain states (Appalachia) with the exception of Pennsylvania and Ohio will go strongly for McDonnell.
The Southwest will be up for grabs and four of these states (CO, NM, NV, and AZ) will be toss-ups until the very end.
The Pacific Ocean states (excluding AK) will vote for Cuomo-Castor. This ticket will be much stronger than Kerry-Edwards and young voters in these three states will favor the Democrats.
I see the states going this way, and for the 2016 election, the electoral votes are changed according to current projections:
Electoral Votes
Cuomo-Castor 271 EVs
McDonnell-Walker 231 EVs
Popular Votes
Cuomo-Castor - 62,191,103
McDonnell-Walker - 58,713,042
Here are the ten closest states and their winning margins:
Wisconsin - 0.23Minnesota - 0.38Arizona - 0.56 Iowa - 0.58Ohio - 0.93New Mexico - 0.98Michigan - 1.15Nevada - 1.28Colorado - 1.48Missouri - 1.74