Elon Musk’s pledge to step back from campaign spending — if he means it — is rippling across the nation’s political landscape. Some Republicans are worried that they might be losing their whale. Some Democrats fear they are losing their foil. It matters because Musk injected an unprecedented level of spending into the presidential race and could do the same in November’s Virginia governor’s race and around the country in the midterms. That was suddenly put in doubt Tuesday, when the...