Paul Ryan has said he does not plan to run for the open Senate seat in Wisconsin. Former governor and HHS Secretary Tommy Thompson does plan to run. Thompson is popular and would give Republicans a strong chance to pick up the Senate seat. on the other hand, he repeatedly endorsed the Affordable Care Act:

Washington, D.C. (November 20, 2009) - Tommy Thompson and Richard Gephardt, Board members of America's Agenda: Health Care for All, today released the following statement on the Senate Health-Care reform bill:
"The health-care bill in the Senate represents another milestone in achieving meaningful health- care reform for millions of Americans. It is now critical that members of Congress work together in a bi-partisan fashion to pass a common-sense, fiscally responsible solution to drive down health-care costs, ensure access to affordable and quality care, increase efficiency and achieve real savings.

That is a huge problem. Many other Republicans endorsed laws like the Affordable Care Act, but then backed away when Democrats decided to pass it into law. Thompson endorsed the Obama plan itself. That's going to be a major liability if a more conservative Republican decides to challenge him in a primary. But it would also be a very significant fact if Thompson manages to win the seat, perhaps if Republicans can clear the primary field for him.

The question is whether Republicans want to take the trade-off of winning a Senate seat in return for accepting a high-profile defector from the anti-Obamacare jihad. My guess is they won't.

If conservatives are looking for a challenger against Thompson, how about right-wing party line enforcer Erick Erickson of redstate.com? The potential Tommy Thompson vs. Erick Erickson showdown would be ideologically and patronymically fascinating. Maybe Pete Peterson could jump in as well.