Once upon a time, the export crop of Martha’s Vineyard and Nantucket was whale oil. Nowadays it is money, harpooned by visiting campaigns from deep-pocketed donors at anchor in their summer seaside retreats. In the peak season of July and August, the islands’ social rhythms are dictated by money-harvesting events, wrangled by local hosts jostling for the social éclat that comes from securing a high-profile honoree. By late August this year, the territory had already been worked over by a series of hopeful visitors with hands outstretched, such as the rising Texas Senate hopeful James Talarico, Hakeem Jeffries in eager...