On July 24, President Obama traveled to Medina, a small, wealthy enclave outside Seattle, to attend a fundraiser for a Democratic super PAC at the home of former Costco CEO Jim Sinegal, one of the Democratic Party’s most reliable donors. But at the same time Sinegal was raising money to help Democrats retain control of the U.S. Senate, he and two of the wholesaler’s top executives were also shelling out cash to help Republicans win the Washington State Senate. In fact, the three Costco executives — long among the most reliable donors to Democratic candidates and causes — have spent...