Former Rep. Barney Frank spent decades as one of the Democratic Party’s most recognizable left-wing figures. Now, in what may be one of his final public warnings, he is telling Democrats that their loudest activists have pushed the party into territory the public simply is not buying. Frank’s warning, delivered as he enters hospice, is blunt. He is not attacking the left from the right. He is saying the left has made a strategic mess of things by turning its most controversial causes into purity tests and then acting surprised when voters balk. “I think we’re in a situation where...