The Obama campaign has already made it pretty clear that they will treat Ryan as Romney's de facto running mate.
Most people don't know who Ryan is though (though of course that will change if Romney picks him as his running mate, which is why he won't be Romney's running mate). This isn't 1996, where Gingrich was the face of the opposition, and was quite unpopular, and you had an endless stream of Clinton ads decrying "The Dole-Gingrich agenda".
Obama will certainly try to tar Romney with the Ryan budget, even if Ryan himself isn't widely known. Which is why Romney won't pick him as his running mate, and he'll make some noises along the lines of "I applaud Rep. Ryan for making the tough choices, which the White House hasn't made, but I'll present a budget next year that will result in similar deficit savings with somewhat different cuts...", and then he won't specify those cuts until after the election.
Poll
