Republicans will make even moderate opponents answer for the radical views of DSA candidates. I have no objection to the Democrats’ nominating an oysterman for a Senate seat. When I led our party’s effort to recapture the House in 2006, we made a point of recruiting candidates with “authentic” broad-based appeal. But that strategy, which flipped 31 seats during my tenure, and claimed 21 more in 2008, was predicated on a substantive premise: We needed to allay voters’ worries that Democrats were weak on national defense and culturally out of touch, vulnerabilities born from previous elections. If we could do...