It will depend alot on Obama's approval ratings/Democrat brand that is.
Scenario 1:
If the economy looks to be recovering, and Obama's policies are seen as a success, barring any major gaffes from either party, and if the GOP nominates Rubio/McDonnell, a close election could look like this:
It all comes down to Ohio. Rubio takes FL 54-46 and VA 51-49, and he takes one of the southwest tri-fecta (CO, NV, NM)
Now, switch out Rubio with Martinez (The GOP would still keep a white male on the ticket). I say they keep MCDonnell because of the swing state prize of Virginia.
Once again, it comes down to Ohio. Here, there is a very narrow win of 270 for the Dems.
Scenario 2:
Obama's policies are preceived as a disaster, and there is a second recession over the last 3 years starting in 2013, and continues to get worse, and by 2016, unemployment is still in the 8-10% range. No matter how much Obama is likable, he is perceived as a failure in the same regard Bush was by 2008. But let's say the Dems counter this by nominating a woman VP (Kobluchar?) with Cuomo as the nominee.
With Rubio, the map could look like this:
With Martinez/McDonnell, the women vote and hispanic vote essentially neutralized, the map could look like this:
