By Ruth Ben-Ghiat

(CNN) -- How does a democratic political culture turn into something more authoritarian? How do leaders persuade people to accept their repressive agendas, scapegoat targeted groups and disrespect democratic norms? In the year since his election as president, Donald Trump has used one strategy to these ends: floating extreme ideas that end up polluting the mainstream.

It works by introducing an idea that is reprehensible to the values of liberal democracy, framing it as an off-the-cuff remark or even as a joke. Later, you can take it back, or blame others for having misheard or misinterpreted you. Meanwhile, your idea has circulated to millions, sometimes even creating its own news cycle. It is in the air. It cannot be unheard.

Some commentators want us to look only at what Trump has actually done for evidence of authoritarian tendencies. They say "it's alarmist" to focus on his threatening speech over his policies. Yet talking (or tweeting) is a form of action: It's a choice to say one thing rather than another, or say nothing at all.

When Trump tweets his congratulations to Chinese leader Xi Jinping on his "extraordinary elevation," and then gushes to an interviewer about Xi's increased power ("Now some people might call him the King of China -- but he's called President"), this is a deliberate message about the kind of leadership he admires and whom he sees as a kindred spirit.

It's easy to miss a key factor in how authoritarianism takes hold: how a leader's comments, little by little, can instill a new vision of what is possible and acceptable. As a professional marketer, Trump understands the power of suggestion. He sees how a casual statement or tweet can become a viral meme that exposes millions to his toxic notions.

Whether it's his remark about the "disgusting" freedom of the press "to write whatever it wants," (which raises the specter of state-mandated media controls) or his remark made "in jest" about Vice President Mike Pence wanting to "hang gays," (which conjures regime-style persecution) he has dangerously expanded the boundaries of public presidential language. It took just two days after the latter remark for anti-LBGTQ posters to appear on college campuses (and Twitter feeds) showing bodies hanging from a noose.

Savvy rulers know that ideas prepare the psychological terrain for future actions, habituating people to consider a distasteful notion as "out there" in the realm of the possible. That's why anti-democratic rulers have always "tested" allies, the public and the media as they consolidate their power, saying things that "push the envelope" against democratic norms to the point of raising the idea of extralegal or irregular action.

Philippine President Rodrigo Duterte used this tactic very explicitly during his 2015 campaign when he vowed to kill thousands of drug dealers and criminals. "I am telling the Filipino people not to vote for me, because it will be bloody," he said. Once he won, he reminded reporters: "I am testing the elite of this country."

It's tempting to dismiss such declarations as bluster. Surely the politician will "calm down" eventually. Yet such reactions merely embolden a leader to be even more reckless. It's at this stage that handlers often cycle through the leader's inner circle, each one trying to contain the fallout from these tests, especially in the foreign policy sphere. In truth, the leader has little incentive to rein himself in: His outbursts play well to his base, and they show the political class that he can't be owned.

Trump has used speeches and Tweets to launch trial balloons for almost two years now, and in the process has changed our expectations for what a leader of a democracy might say and do. In typical fashion, one of his strongest and most revealing "testing" statements came very early on.

In January 2016, he boasted that he could stand on Fifth Avenue and shoot someone without losing any followers. Most Americans filed this away as "Trump being Trump," but it was a milestone in Trump's process of gauging the strength of our commitment to prevailing political norms. With one tossed-off remark, he tested the loyalty of his base, and the GOP's tolerance for a candidate who claimed the right to act with impunity in potentially many areas of governance. Robert Mueller's ongoing investigation takes on such attitudes, which the GOP never felt the need to rebuke.

Authoritarianism is not something presented to us as a fait accompli, but something we help along, step by step, by acquiescing to changes in political climates that start with pronouncements by the leader and slowly move the boundaries of what is possible.

Leaders such as Trump use such unsettling language to demoralize and frighten the populace and incite greater political and social polarization that, in the end, benefits only them. Trump has shown he's eager to help us lose our conscience and our principles: It's up to us to let him know we are wise to his game, and will stand our ground.