If you're going to run against Paul Ryan, you might as well run against Paul Ryan:

Rob Zerban thinks he can knock off a Wisconsin giant next fall.
And Democrats on Capitol Hill agree that this 42-year-old businessman, a former Republican with limited experience in county politics, could be their best shot at defeating House Budget Chairman Paul Ryan (R) in more than a decade.
Democrats certainly believe they have an opportunity to nationalize the Wisconsin House race, driving resources and energy from across the country into a swing district Ryan has held relatively comfortably since 1998. And they believe that fallout from New York’s GOP debacle will help.

Ryan has always won handily, but he's never faced serious opposition before. His district has a Partisan Voting Index of Republican +2, making it just slightly more Republican than the country as a whole. George W. Bush won 53% of the vote there in 2004, and Barack Obama won 51% in 2008. Given the deep unpopularity of Wisconsin governor Scott Walker, not to mention that of Ryan's plan itself, it hardly seems inconceivable that a strong challenger could knock Ryan out of the House of Representatives.