Rick Santorum had been the last best hope of Christian conservatives who opposed Mitt Romney, derided by many as a “Massachusetts moderate.” Now, facing the abrupt end of Mr. Santorum’s presidential bid, some evangelical leaders have begun to rally behind Mr. Romney, saying their shared hostility toward President Obama will be a powerful force for uniting the party in November. In one sign of coalescing support from Christian conservatives, the National Organization for Marriage, a leading opponent of same-sex marriage, endorsed Mr. Romney on Wednesday morning. The group called Mr. Romney a “true champion” and said that President Obama “has...