Ted Cruz (R., Texas) has been a United States senator for only 34 days, but already he is making his mark on national politics. His conspicuous presence and aggressive tone have thrilled his conservative cheerleaders, while inducing fits of rage in liberal detractors and Joe Scarborough. In the past week alone, Cruz has tangled with veteran Democratic spin-master Chuck Schumer (D., N.Y.) on Meet the Press, sent a tongue-in-cheek letter to Mayor Rahm Emanuel of Chicago, introduced legislation to fully repeal Obamacare, and recorded “no” votes on major items, including Hurricane Sandy relief, raising the debt ceiling, filibuster reform, and...