Andrew Coan is a professor of law at the University of Arizona and author of the forthcoming book , "Prosecuting the President: How Special Prosecutors Hold Presidents Accountable and Protect the Rule of Law." The views expressed here are the author's. View more opinion articles on CNN.

(CNN) In recent weeks, a new consensus has emerged among many legal analysts and observers: Special counsel Robert Mueller is finally closing in on President Trump. This could be correct, but at present, it is still mostly conjecture. There remains a real possibility that Mueller will come up empty, at least in connection with President Donald Trump, his family, and high-level advisers.

Neither liberals nor conservatives have adequately considered this prospect. They should.

For liberals, it is an article of faith that Mueller will produce a damning report if only he is allowed to complete his investigation. Conservatives dispute this, but their incessant attacks on Mueller suggest a far more pessimistic outlook. Why try to destroy the man who is about to exonerate you?

This view is broadly shared for a reason. By historical standards, Mueller has run an extraordinarily effective and efficient investigation. He has already indicted dozens of Russian nationals for meddling in the 2016 election. More important, he has negotiated plea agreements with senior officials from Trump's campaign, including Paul Manafort's protégé, Rick Gates, and retired Lt. Gen. Michael Flynn. Experienced prosecutors enter such agreements only when a witness can offer significant ammunition against higher-value targets.

But Mueller himself has been almost totally silent. Outside the special counsel's team, no one really knows what evidence he has found or what conclusions he has drawn. Many of the possible crimes he is investigating turn on questions of intent that are legally murky and notoriously difficult to prove. It is impossible to rule out the possibility that the investigation will end with a whimper, rather than a bang.

If it does, liberals are sure to be despondent. Many of them have been counting on Robert Mueller to rescue them from the waking nightmare of Trump's presidency. As long as he is on the case, they have had at least one reason for hope. Conservatives, by contrast, will feel gleefully vindicated. It is easy to imagine Trump gloating for weeks, even years, at his rallies and on Twitter.

Both sides would be wrong.

If Mueller completes his investigation without further interference, that in itself will be a victory for the rule of law. Trump has tried mightily to shut the special counsel down. That he has failed thus far is already something of a miracle. More to the point, it is a testament to the resilience of American institutions under extraordinary strain

Mueller's defenders should insist upon this. All along, their rallying cry has been the principle that no one, not even the President, is above the law. When the President and his close advisers are credibly implicated in criminal activity, this principle demands a searching and independent investigation, free from political interference. If Mueller succeeds in completing such an investigation, the rule of law will have prevailed, whatever the result. That is something to celebrate.

Won't this also vindicate Trump? It will not. At most, it will establish that he is probably not a criminal -- at least so far as Russian election meddling is concerned. (Campaign finance violations are another story, as Michael Cohen's sentencing memo makes clear .) It will not establish that the investigation was a witch hunt or even a waste of time. The whole point of an investigation is to find answers. Without Mueller's investigation, we would never know if the credible allegations against Trump and his associates were true.

Most important, the rule of law will have prevailed despite Trump's relentless efforts to undermine it -- and only by a hair's breadth. No matter what happens, Trump's behavior sets an ominous precedent. For better or worse, the democratic norms that make up the fabric of our constitutional order are fashioned from -- or eroded by -- historical episodes like this one.

When Ulysses S. Grant fired the first special prosecutor in US history in 1875 , norms of presidential conduct remained weak for decades. When Richard Nixon fired Watergate special prosecutor Archibald Cox, the public backlash forced him to appoint another genuine replacement, who ultimately forced Nixon's resignation. This episode served as a cautionary tale restraining presidents for two generations.

Stay up to date... Sign up for our new newsletter. Join us on Twitter and Facebook

If Mueller completes his investigation without interference, the norm established by Watergate will remain superficially intact. But Trump will have rendered it far weaker and more fragile than it appeared just two years ago. The next time a president is accused of a crime, this weakened norm may not be enough to restrain him. For this, Trump deserves lasting censure, regardless of the investigation's outcome.

If there was no collusion and no obstruction, the irony is that Trump could have avoided all of this -- the damage to the rule of law and to his presidency -- by cooperating fully and letting Mueller's probe run its course. If future presidents take any lesson away from Trump, let us hope it is this one.