By now it’s become a disturbingly familiar pattern for conservatives: A candidate who seems promising at first glance wins a Republican primary, and then suddenly the press — often fed by a Democratic rival’s opposition-research team — begins looking in-depth at every controversial and regrettable statement and act that candidate ever made. Just days after it’s too late to change the nominee, the choice of GOP primary voters appears to be an egregious mistake. The most recent, and perhaps most extreme, example of this phenomenon is Kentucky Senate candidate Rand Paul. Overnight, Paul changed from a soft-spoken, genially libertarian version...