Democrats kept their impeachment articles limited to the Ukraine affair, but that decision was fraught.

For much of the summer and fall, Jerrold Nadler, the chairman of the House Judiciary Committee, openly flirted with moving to impeach Mr. Trump on charges of obstruction of justice stemming from Robert Mueller’s investigation into Russian election interference. Mr. Nadler’s panel drew up subpoenas for Mr. Trump’s closest advisers and held hearings on the topic, with witnesses including Corey Lewandowski, Mr. Trump’s former campaign manager, and constitutional scholars.

In their report, Mr. Mueller and his prosecutors seemed to agree that Congress might have a role in addressing Mr. Trump’s obstruction, in the absence of formal charges. “A federal criminal accusation,” the report says, could “preempt constitutional processes for addressing presidential misconduct.” In a May news conference, Mr. Mueller said that the Constitution “requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing.”