Mark Leibovich describes the idea (via Sullivan):

Ryan told me that he envisions a vice presidency in which he oversees a vast portfolio on fiscal and economic issues. He agreed fully with the suggestion — first mentioned to me by one of his advisers — that his assignment could be the fiscal version of the role Dick Cheney played on national security after Sept. 11.

I don’t doubt that this is what Ryan boosters have hoped for when he was selected. The hope that Ryan could decisively influence a Romney campaign and future administration is what seemed to be behind Ryanmania in the weeks before August 11. Ryan may think that this is what will happen, but I suspect that it won’t, or at least not in the way that is being described here. As I said two months ago:

The main problem with this is that Ryan will be very much Romney’s junior partner rather than acting as an eminence grise. It is more likely that Ryan would be wasted in the VP role, and within a year or two Ryan’s boosters would probably be writing wistful columns entitled “What Happened to Paul Ryan?” and “Where Did the Real Paul Ryan Go?”

We have already seen how Ryan has conformed himself to Romney’s campaign. Just as Ryan fell in line as a “team player” for the campaign, we should assume that he would do the same in a Romney administration. If Ryan received a portfolio to handle fiscal and economic policy matters, as he probably would, it is doubtful that he would be able to have such a huge influence on administration policy as Cheney did. Just as it has been in the campaign, the reason for having Ryan take the lead on fiscal matters in a Romney administration would be to create the impression that Romney takes fiscal responsibility seriously without having to be fiscally responsible. It is more likely that Ryan’s practical function would be to defend and justify administration fiscal policy to skeptical conservatives while serving as a lightning rod for criticism from the left.