There can be little doubt that Mr. Trump views the Justice Department as a tool to punish his political enemies and give his allies a free pass. He has also made it clear that he hates “rats,” praising the felon, Paul Manafort, who once ran his campaign for refusing to “break” by cooperating with law enforcement.

This is our new normal, and congressional Republicans have made it clear through their inaction that they will not remove Mr. Trump from office for obstruction of justice. That matters because the Justice Department has long concluded that a sitting president cannot be indicted (although for many legal scholars, that remains an open question). Mr. Mueller is likely to follow that guidance and submit a report to Congress instead of indicting the president for obstructing justice.

It is hard to imagine 19 Republican Senators voting to convict Mr. Trump for obstructing justice. In itself, that does not mean his presidency will survive until 2020 (or beyond), given the array of legal problems he now faces. After all, just two weeks ago, his former lawyer, Michael Cohen, told a judge under oath that Mr. Trump directed him to commit crimes. But obstruction appears to be the first existential challenge to the Trump presidency if reports that Mr. Mueller is preparing a report regarding obstruction are accurate.

On the surface, the Senate’s refusal to remove the president from office for obstructing justice does nothing to counteract the other legal problems that he faces, such as the investigation of Mr. Cohen that resulted in immunity for Allen Weisselberg, the Trump Organization’s chief financial officer.

Despite Mr. Trump’s rhetoric and threats to impede the Mueller investigation, warnings from his team that his acts could create liability for him seem at times to have restrained the president. For example, Mr. Trump reportedly did not carry out his plans to fire Mr. Mueller in the face of pushback from Don McGahn, the White House counsel. More recently, the president did fire Mr. McGahn 11 days after reports emerged that the White House counsel had cooperated with Mr. Mueller’s investigation. But the president followed up — presumably because of warnings from Mr. Trump’s attorneys — with corrective tweets claiming that the firing was not punishment for Mr. McGahn’s cooperation.