Sounds far-fetched and, to some, totally implausible. But the Republicans are losing potential candidates at a pace that is downright alarming if you believe in a healthy two-party system. The demise of John Ensign’s political career a few weeks ago and the surreal downfall of Mark Sanford last week is enough to send chills through the even the most optimistic Republican strategist. We know that of the 2008 crop, only Mitt Romney seems likely to stay on as a contender. The old stalwarts like Newt Gingrich may get a lot of press, but it is unlikely they can mount a...