Jeannie Suk Gersen writes on the obstruction-of-justice charges against Presidents Richard Nixon and Bill Clinton, and why the case against Donald Trump, in the wake of the Mueller report, might be distinct.As we enter a new phase of heightened hostilities between the President and the Democratic House, there will be myriad opportunities for Trump to commit further process offenses, including contempt of Congress. Those offenses may well push Congress toward more serious impeachment discussions, in order to defend constitutional separation-of-powers principles. Mueller anticipated this, in his report, with a clear statement: “The conclusion that Congress may apply the obstruction laws to the President’s corrupt exercise of his powers of office accords with our constitutional system of checks and balances and the principle that no person is above the law.”