There are 270 electoral college votes needed for victory. Looking at the map and the baseline votes, the optimistic view suggests Republicans will need to gain 100 electoral votes beyond their solid red states (safe and likely). The "leans" Republican and "toss up" states total to 121 votes, if every one of those is won by a Republican nominee.
This is going to be extremely difficult and challenging, with or without a strong GOP nominee. Here's why. Several of those states are union states, which will fight to the death for Obama. They are going to focus on those states heavily with money and their SEIU thug tactics. Another factor will again be retirement states such as Florida and North Carolina. Those two toss up states alone comprise 44 electoral votes. It is close to impossible, basing estimates on this baseline, to see a Republican 2012 victory without those two states.
The good news here is that we have some newly-elected Republican governors in the union states and in Florida. If they are building strong GOP state chapters and the RNC is focusing on helping them organize, this could help a Republican nominee. Also, there are some redistricting factors that haven't even been realized yet and they could play heavily into the favor of the Republicans.
Bottom line: if the GOP puts up a weak candidate, who can't generate enthusiasm for a GOP "big tent" by reaching conservatives and independents, then the optimistic scenario is completely unattainable. Polarizing candidates won't win the electoral votes needed in the toss-up states. Difficult and controversial issues (entitlement spending and the national debt) are going to have to be treated with kid gloves by the GOP nominee. He or she will have to swing hard at Obama and present realistic and maybe even reserved solutions. It can't be emphasized enough how important it will be to select a candidate who can win. It may not be the favorite of the Tea Party or the establishment GOP or the independents or the social conservatives, etc..., but a Republican who can unseat Obama will certainly be my favorite.
I won't go into the gory analysis of the pessimistic viewpoint of the electoral votes needed by the GOP in 2012. I think the "optimistic" view sets a difficult enough tone by itself.