It was just one sentence, uttered to reporters who had gathered around Representative Ilhan Omar of Minnesota last week when she turned up at a Columbia University encampment to offer a show of support for pro-Palestinian protesters — among them, her daughter, a student activist — demonstrating against the Israeli attacks on Gaza. Ms. Omar, one of the leading pro-Palestinian voices in Congress, rejected the argument that the protests were antisemitic, noting that many of the participants were Jewish. “All Jewish kids” should be kept safe, she said, no matter which side they were on in the debate — or,...