Despite Mr. Mueller’s unwillingness to speculate on hypotheticals, and his adherence to the Justice Department policy against indicting a sitting president, these facts, which he also outlined in depth in his report, make clear that were Mr. Trump an ordinary person, he would have been indicted on multiple counts of obstruction of justice, as more than a thousand former federal prosecutors, free of those limitations, have observed.

Republicans did their best, during their own questions, to muddy these facts by attacking Mr. Mueller’s credibility. And of course from the beginning, Mr. Mueller was limited by a finding by the Office of Legal Counsel that a sitting president could not be indicted. And yet there is no doubt, after Wednesday, that Mr. Mueller found substantial evidence of serious and repeated criminal activity by the president. Inaction is not an option. So what next?

Wednesday’s hearings are significant because they mark the point where Congress will take up where Mr. Mueller left off; they were, effectively, the beginning of an inquiry into whether these and other abuses of power are the “high crimes and misdemeanors” that the Constitution says would merit President Trump’s impeachment and removal from office. Congress needs to continue that effort and make it official.

To call for a formal impeachment inquiry to begin is not to prejudge its outcome. It is to acknowledge that the preconditions for Congress for initiating that process have undoubtedly been met. It is also an acknowledgment that as a nation we have exhausted other avenues for accountability.

Since his inauguration, Mr. Trump has made clear that he will not abide by the normal standards of ethical and responsible government. Since Democrats took control of Congress in January, he and his administration have asserted baseless privileges and immunities to undermine ordinary forms of legislative oversight of the executive branch, attacked all attempts at review or accountability, and ignored recommendations to discipline top officials for repeatedly violating the law.

President Trump has threatened to dismantle the proposition that no person is above the law. But given what we have all now heard from Mr. Mueller, it is clear that Congress must assert itself as a constitutional protector of the rule of law.

In opening an impeachment inquiry, Congress must confront other questions that Mr. Mueller was not asked to answer — the most important of which is whether President Trump has engaged in other constitutionally repugnant abuses of power. In addition to obstruction of justice, there are at least two more, independent bases for impeachment that Congress should assess.