Donald Trump thinks approval ratings are rigged. That’s scarier than you think.

There are woefully few people and institutions capable of meaningfully constraining Donald Trump.

Congress won’t do it; Paul Ryan and Mitch McConnell have made it clear they’re not interested in pressuring the president-elect on his sundry conflicts of interest or in launching investigations into Russian involvement in the election. Opportunities for state-level resistance are limited, given the small number of states with Democratic governments. Foreign countries will not have much influence; at the end of the day Germany’s Angela Merkel has few tools to push Trump into line with Western Europe, and the UK’s Theresa May and France’s François Hollande are in even worse positions to pressure him.

But one of the most important checks on the president, on any politician, is public opinion. While it may not always feel true, political scientists broadly agree that presidents have historically been quite responsive to public opinion, perhaps more so than the framers — who saw the office as standing somewhat aloof of mass sentiment — originally intended.

Public opinion is a valuable constraint on the president’s freedom of action. It prevents the president from abusing the unilateral powers of the office to benefits cronies and political allies; it helps explain why Richard Nixon and Gerald Ford never pardoned the Watergate burglars and conspirators (besides Nixon himself), why George W. Bush didn’t preemptively pardon members of his administration who engaged in torture, why corrupt members of Congress from a president’s own party still face prosecution.

It has forced major changes in foreign policy too. The public opinion rebuke of a lukewarm New Hampshire primary performance pushed Lyndon Johnson to withdraw from the presidential race, halt bombing in North Vietnam, and commence peace talks with the country. Cratering approval ratings and the 2006 midterms forced Bush to fire Donald Rumsfeld and try a new strategy in Iraq.

Which all makes it quite disturbing that Donald Trump appears to believe approval ratings are rigged. First, before inauguration, he tweeted this:

Then, after CNN aired a segment on a poll they conducted showing public opposition to the Trump administration’s efforts to bar admissions to the US by citizens of seven Muslim majority countries, Trump declared all negative polling “fake news”:

Approval ratings and other polls aren’t the only way public opinion can constrain presidents; midterm and primary elections can do the trick too, as can rallies and protests and other displays of activism. But approval ratings are an undeniably important mechanism. They’re a leading indicator, accessible in nearly real time, with a clear connection to the president’s own material self-interest. A low approval rating means a lower chance of getting reelected, and a higher chance of losing seats in Congress and even state legislatures; there’s a reason presidential approval is a key variable in the most accurate predictive models for presidential and down-ballot elections. Beyond being bad in and of itself, it is bad in a real, concrete way for the president personally and, often, for the rest of his party.

So if Trump does dismiss these numbers, that means he’s ignoring one of the only things that can hold him back. Instead of taking polls as indicators — imperfect indicators, but still — that something is going wrong and he might want to pivot for his own good, he might not consider them at all. If he abuses the pardon power and his favorability plummets, he won’t notice. If he repeals Obamacare without replacing it and the country rebels, he won’t believe that Americans view that as a mistake.

This wouldn’t necessarily be abnormal behavior. There’s a difference between how presidents behave when they have relatively high or low approval ratings — and Trump is already the least popular president-elect in recent history, by a large margin — and how they behave when they have average approval. In the latter circumstance, they have an incentive to align with public opinion to boost those ratings further and bolster their chances in future elections. But Princeton’s Brandice Canes-Wrone and Stanford’s Kenneth Shotts have found that presidents with very low or very high approval ratings are less responsive to public opinion than presidents with average approval.

Extremely popular presidents seem to feel more politically secure, and can afford to break off from public sentiment when they disagree with it. At the other extreme, very unpopular presidents may figure they’re doomed anyway, and that they might as well do what they like and hope it’s such a massive success that it works out. Trump seems to be finding himself in the latter camp.

Unlike presidents like Bush or Nixon, who found themselves massively unpopular toward the end of their tenures, Trump is beginning his term that way. So even though his unpopularity suggests that the nation wants checks on him, and thereby likely wants public opinion to constrain him, Trump’s own statements and the experience of history suggest that the worse his ratings get, the more likely he’ll be to dismiss them.