Congress must struggle to function in these inquiries with the necessary secrecy, professional resources and time. It must do all this while containing internal partisan political conflicts and pressures. And it must act while mindful of the sensitivity of a legislative probe into an executive’s motivations when performing otherwise constitutionally authorized acts.

Impeachment is not a substitute for the criminal justice system. In cases like this, it requires the support of that system — the results of professional investigation and judgment.

The attacks on Mr. Mueller, together with Mr. Trump’s lawyers’ embrace of an aggressive legal theory, put in question whether the Department of Justice will be permitted to complete its investigation. Should the president next take the step of firing Mr. Mueller, he would end professionally conducted fact-finding on this crucial question of motive. Congress is then left to respond by trying to complete the department’s job or to proceed to impeachment and a showdown over which branch can claim primacy for its constitutional functions. It faces major challenges in accomplishing the first task, and the second course may lead to constitutional conflict in which, upon impeachment and conviction, the president refuses to vacate his office and the Supreme Court is called on to settle the dispute. Call it Bush v. Gore, the Sequel, another make-or-break case deciding who will be — or, in this instance, remain — president.

The preservation of presidential accountability to the rule of law requires the protection of the department’s role. If the professional prosecutors conclude that they will not bring a charge of obstruction, Congress will have the benefit of the evidence accumulated in the investigation in making its own judgment about impeachment.

Or the special counsel may determine that indictment is justified and proceed to prosecute, which will also force the issue of whether the president can be indicted while in office. The president is highly likely to argue the case for his immunity, but he should have no confidence that he will succeed. Beginning with United States v. Nixon, through later decisions in the Clinton years against presidential claims of privilege or of immunity from civil suit, the courts have stood firmly behind the proposition that the president is not above the law.

Whether or not an investigation concludes with an indictment, Congress will gain the tested evidence that it needs to assess the basis for impeachment for obstruction. Congress and the department would each be able to do what they are cut out to do. Nothing prevents criminal investigations and impeachment processes from running concurrently, as they have in the past.

Mr. Trump and his lawyers should not prevail on a position that puts him beyond the reach of criminal investigation while seriously undermining Congress’s capacity for evaluating the grounds for impeachment. These are the stakes in the battle over the future of the special counsel’s investigation.