Paul Ryan may be the most useful Republican House member in decades when it comes to domestic policy, but I think that Ryan (based on the tax plan in his original Roadmap) is fatally flawed as a presidential candidate. If someone has a good rebuttal to the math in the link, I'd love to see it.
I'm impressed by Rubio's rhetoric (usually), but a presidential candidate who is a state legislator turned Senator with less than a year of service is a problem for several reasons. Republicans need a more politic plan for a sustainable budget than anything Ryan has offered, and need a candidate with a record of executive competence so the public will be reassured that the person offering these radical-sounding (if gradual) changes isn't some ideological fantasist who doesn't know how to implement policy in a responsible way.
So you say that Obama is a state legislator turned freshman Senator and he is now President. Sure, but, on policy, the Republican challenge in 2012 is almost the reverse of Obama's challenge in 2008. Obama basically ran by promising everything to everybody at the low, low price of some tax increases on the top 2% of earners. His lack of a record helped because he hadn't been in the Senate to vote for a bunch of tax increases (or against tax cuts he now said he wanted to keep) and defense cuts.
Republicans, if they are to be at all responsible, are going to have to offer a plan for fiscal consolidation that will touch the lives of tens of millions. It isn't going to sound good. It is going to sound disruptive and scary. The Republicans will have to convince the marginal voters that the Republican plan is preferable to the combination of tax increases and centralized health care cuts that will come if Obama is reelected. This is why a record of maintaining core government services while cutting spending down to a sustainable level would be a key asset for a 2012 Republican presidential candidate.
Yeah, I know it is a one-in-who-knows-what-astronomical-number chance.