Or if Mr. Mueller subpoenas Trump to testify and Mr. Trump defies the subpoena, it's a constitutional crisis.
Or if Mr. Mueller comes up with substantial evidence that Mr. Trump is guilty of colluding with Russia or of obstructing justice but the House doesn't move to impeach him, we'll have a constitutional crisis.
I have news for you. We're already in a constitutional crisis. For a year and a half the president of the United States has been carrying out a systemic attack on the institutions of our democracy.
A constitutional crisis does not occur suddenly like a coup that causes a system of government to collapse. It occurs gradually, as that system is slowly weakened.
The current crisis has been unfolding since the waning days of the 2016 campaign, when Mr. Trump refused to say whether he'd be bound by the election results if Hillary Clinton won.
It continued through March 4, 2017, when Mr. Trump claimed, without evidence, that Mr. Obama had wiretapped his phones in the Trump Tower during the campaign.
It deepened in May 2017, when, by his own admission, Mr. Trump was thinking of "this Russia thing" when he decided to fire FBI Director James Comey, who had been leading the bureau's investigation into Russian meddling in the 2016 election, and then admitted to Russian officials that firing Mr. Comey had relieved "great pressure" on him "because of Russia," according to a document summarizing the meeting.
A constitutional crisis becomes especially dangerous when a president of the United States tells the public it cannot trust the government of the United States.
Over the last few weeks, Mr. Trump has done just this.
First he accuses the FBI of sending a spy to secretly infiltrate his 2016 campaign "for political purposes." Then he "demands" that the FBI investigate the spying -- resulting in the Justice Departmentsharing portions of the FBI investigation with Mr. Trump's allies in Congress.
Mr. Trump blames the entire Mueller investigation on a conspiratorial "deep state" intent on removing him from office. He uses pardons to demonstrate to those already being investigated that they shouldn't cooperate because he can pardon them, too.
He claims he has the absolute right to pardon himself and can thereby immunize himself from any outcome, and he asserts that he has the power under the Constitution to end the investigation whenever he wants.
The constitutional crisis worsens every time Mr. Trump berates judges who disagree with him, attacks intelligence agencies that won't do his bidding, and calls journalists and news organizations that criticize him "enemies of the people," and their reporting "fake news."
It deepens when he avoids news conferences and instead communicates with his followers through tweets and rallies. And when he treats Americans who didn't vote for him or who disapprove of him as his personal opponents, rather than as citizens to whom he is as constitutionally accountable as to his most loyal supporters.
It intensifies when he uses the presidency as a personal fiefdom to enrich himself and his family; unilaterally breaks treaties and starts trade wars with long-standing allies; and expresses admiration for some of the most murderous dictators in the world.
The crux of America's current constitutional crisis is this: Our system of government was designed to constrain power, but Mr. Trump doesn't want to be constrained.
Our system was conceived as a means of promoting the public interest, but Mr. Trump wants to promote only his own interest.
Our system was organized to bind presidents to the Constitution, but Mr. Trump doesn't want to be bound by anything.
The crisis will therefore worsen as long as Mr. Trump can get away with it. A megalomaniac unconstrained by countervailing power becomes only more maniacal. He will fill whatever political void exists with his unbridled ego.
The only legal way to constrain Mr. Trump is to vote for a Congress this November that will stand up to him. And then, in November 2020, vote him and his regime out of office.
If he refuses to accept the results of that election, as he threatened to do if he lost the 2016 election, he will have to be forcefully removed from office.
Friends, we are no longer trying to avert a constitutional crisis. We are living one. The question is how to stop it from destroying what's left of our democracy.
Robert Reich, a former U.S. Secretary of Labor, is professor of public policy at the University of California at Berkeley. His latest book is "The Common Good.” His documentary, "Saving Capitalism," is available on Netflix.