There are many factors to consider as to how Ryan on the ticket changes the political calculus. One is the October debates.
My idea that the presidential debates will be crucial and will push polls in Romney's direction (and enable him to win the election) carries less weight now than previously, I think. The Ryan pick ensures heightened attention to the Oct. 11 VP debate, and Ryan enthusiasts are touting how he'll wipe the floor with Joe Biden. I'm not so sure. The event might highlight Ryan's lack of foreign policy experience. But in any case, I don't see how the Ryan pick enhances Romney's prospects in the debates. If Romney shows up offering Ryan's fiscal ideas, he looks weak and derivative. If he doesn't, he wastes the supposed advantage in "ideas" that the Ryan pick gave him. The upside is slim, it seems to me.
As for Obama, his debate strategy now becomes talking about Ryan's plans more than about other attack points such as Bain--but certainly not about his own plans and record in any case.