Democrats don’t like to think about Republicans if they can help it. But for those on the left now contemplating their own “progressive tea party” movement, they’d do well to contemplate one woman in particular: Christine O’Donnell.

In 2010 Republicans had an unusual opportunity to win the deep-blue Delaware Senate seat once held by Joe Biden. The vessel: GOP Rep. Mike Castle, a popular former governor and the front-runner. Enter Ms. O’Donnell. Fueled by tea-party enthusiasm and money, she trashed Mr. Castle as a liberal and...