The clock is ticking for special counsel Robert Mueller. With six months to go until November’s midterm elections, Mr. Mueller’s investigation into Russian meddling in the 2016 presidential campaign will soon run into a dead zone of sorts, in which former prosecutors say they expect him either to wrap up, or lie low and take no visible steps until after the election. Though Mr. Mueller doesn’t face any specific legal deadline, the fall midterms amount to a political one, according to experts and prosecutors. He will reach a point this summer when Justice Department habits dictate that he will have...