Americans enjoy a constitutional democracy where criticism is protected and respected. That certainly doesn’t mean political leaders have to agree with their critics, but it should mean that they understand criticism and dissent are part of our democracy.

Donald Trump’s response to his critics suggests he does not understand or accept this cornerstone feature of American society. Rather than address the substantive points critics make (which would be entirely legitimate), Trump instead attempts to delegitimize or personally discredit those who disagree with or question him.

Consider:

When former U.S. Secretary of Defense Robert Gates wrote an op-ed recently setting out his conclusion that Trump is “unqualified and unfit to be commander-in-chief,” Trump resorted to ad hominem, unsubstantiated attacks, calling Gates “a nasty guy” who “[p]robably has a problem we don’t know about.”

When New York Times op-ed columnist Maureen Dowd expressed concern about violence at Trump rallies, Trump tweeted that Dowd is “crazy” and “[a] neurotic dope!” while threatening to sue the Times based on a made-up legal claim not recognized by the law.

When parents of a slain U.S. military officer expressed their political opposition to Trump, the Republican candidate responded by insulting them and claiming they had no right to attack him (in fact, the U.S. Constitution protects this right).

When Trump doesn’t like the way reporters are covering his campaign, he has barred them from his events and placed them on a blacklist.

“ The Republican presidential nominee would find much to like in a system that stamps out dissent ”

Indeed, Trump’s response to his own critics suggests that the Republican presidential nominee would find much to like in a system that stamps out dissent — such as Russia now under President Vladimir Putin, its authoritarian ruler.

It is well known that Trump has frequently expressed praise for Putin. Trump described a compliment from Putin as a “great honor” from a leader who Trump claims is “highly respected in his country and beyond.” Trump applauds Putin for exercising ”very strong control” over Russia and for being, in Trump’s estimation, a stronger leader than President Barack Obama.

Trump attempts to distinguish his praise for Putin from his views of Russia’s political system (Trump claims not to like the latter). In truth, it is difficult to separate Putin from the system he largely controls.

One of the things Trump has praised Putin for is the Russian autocrat’s popular support, expressing admiration for Putin’s 82% approval rating. What Trump apparently does not consider is how Putin has built this support.

Read: How Donald Trump gets away with birther conspiracy and other lies.

As Washington Post columnist Philip Bump notes, Putin “has zero tolerance for dissenting media opinions.” Since Putin first came to power in 2000, “34 journalists have been murdered in Russia.” Putin exercises tight control over the media. Freedom House, which rates Russia’s press status as “not free”, has reported “an increase in propagandistic content in the Russian news media and tighter restriction on dissenting views.”

Putin declares victory in Russia's parliamentary elections

Those independent Russian media outlets that have managed to survive face increasing pressure from the state. Freedom House has further noted that there is a “persistent threat of deadly repercussions for expressions of dissent” in Russia.

It’s not only Russia’s journalists who worry about pressure from the government; ordinary citizens are scared too. One Russian woman who was recently interviewed by the New York Times expressed concern that Putin will not permit a real opposition party. She refused to provide her full name, the Times report noted, “for fear of a visit with Russian state security, the F.S.B.” The Times reports that Putin “long ago reduced [the Russian Parliament or Duma] to a cheerleading squad that endorses his every move.” One opposition candidate says that “anyone who disagrees, or even does not applaud [the Putin government], is labeled an enemy and a fifth column.”

There is no way to say with certainty how Trump would act as president, but Americans ought to take his words and actions seriously, as they are the best evidence as to what he might do.

What we see is a man who, like the Russian leader he admires, rejects the very idea of criticism and dissent. This is not a healthy trait in a president; in fact, it suggests Trump does not understand the fundamentals of American democracy.

Chris Edelson is an assistant professor of government in American University’s School of Public Affairs. His latest book isPower Without Constraint: The Post 9/11 Presidency and National Security(University of Wisconsin Press).