Romney has made the risky but defensible calculation that, if he is to concur most of his party’s ideological baggage, he might as well bring aboard its best salesman. And Ryan is that. During his rise to power he has displayed an awesome political talent. He is ambitious but constantly described by others as foreswearing ambition. He comes from a wealthy background but has defined himself as "blue collar," because he comes from a place that is predominantly blue collar. He spent the entire Bush administration either supporting the administration’s deficit-increasing policies, or proposing alternative policies that would have created much higher deficits than even Bush could stomach, but came away from it with a reputation as the ultimate champion of fiscal responsibility.
What makes Ryan so extraordinary is that he is not just a handsome slickster skilled at conveying sincerity with a winsome heartland affect. Pols like that come along every year. He is also (as Rich Yeselson put it) the chief party theoretician. Far more than even Ronald Reagan, he is deeply grounded is the ideological precepts of the conservative movement – a longtime Ayn Rand devotee who imbibed deeply from the lunatic supply-side tracts of Jude Wanniski and George Gilder. He has not merely formed an alliance with the movement, he is a product of it.