In the NFL, savvy teams oftentimes seek out a backup quarterback whose play style mirrors that of their starter — the idea being that in the event of an injury, their offense can continue to operate as it’s meant to. Maine Democrats have followed a similar course by tapping one sleazy, antisemitic goon to replace another. After the long-foreshadowed implosion of Graham Platner’s campaign for US Senate this summer, progressives demanded that he be replaced by a candidate just as regressive and demagogic. They found their man in Troy Jackson, the former president of Maine’s state senate who had been...