‘Given my experience working for Mr. Trump,” Michael Cohen told the House Oversight Committee last week, “I fear that if he loses the election in 2020 that there will never be a peaceful transition of power.” This scenario—an embattled president suspending the Constitution to cling to power—is a perennial urban political legend, especially if the president is a Republican.

In 1962 two political columnists, Fletcher Knebel and Charles W. Bailey, published a bestselling novel, “Seven Days in May” about an unsuccessful coup...