From my conversation with Jay Cost (who confesses he likes to keep his head down during primaries):

LOPEZ: Can Paul Ryan push his plan and save the seats of members who stand with him?
COST: It won’t be up to Paul Ryan. Instead, it will be about the 2012 Republican nominee, who will be the one to make the case for the need for major changes.
An important point to keep in mind: Right now, 14 percent of all Americans are on food stamps. Nearly two out of every ten personal income dollars are coming from the government. Republicans cannot just run on entitlement reform. They have to run that as part of a package, one that emphasizes getting the private economy working better. Paul Ryan, as chairman of the House Budget Committee, is not in a position to convey that broader message.
But the GOP nominee will be. That’s where the whole game is.
LOPEZ: Could Paul Ryan be that nominee?
COST: He could. The last House member elected to the White House was James A. Garfield in 1880. He was a dark-horse candidate to emerge out of a protracted struggle between the Stalwart (who wanted to nominate Ulysses S. Grant for a third term) and Half-Breed (who didn’t) factions in the GOP. So it is very rare. But I think the key limitation for House members seeking the White House — lack of a national profile — does not apply to Ryan.
Of course, another limitation does. By the time a House member acquires that kind of national profile, he or she also probably possesses one of the major power positions in the institution, meaning that staying in the House is an attractive option. Chairman of the Budget Committee is one such position. Does Paul Ryan really want to leave that post to go campaign for the next 20 months? The upside is that he could be president. The downside is that he might wind up with nothing. And if he doesn’t run, he’s bound to be a player in 2013 and beyond — the Republicans are likely to hold the House, meaning that he’ll be integral to the discussion moving forward, regardless of what happens in the presidential election.
If I were in his shoes, I’d stay put.

More on Palin, Romney, Bachmann, Pawlenty, and more here.