Donald Trump in Radford, Va., on Monday. (Chris Keane/Reuters) (Chris Keane/Reuters)

SOME READERS ask how Donald Trump can be a threat to democracy if he is putting himself forward as a candidate. If he ends up attracting a majority of American voters, what could be more democratic?

First, you don’t have to go back to history’s most famous example, Adolf Hitler, to understand that authoritarian rulers can achieve power through the ballot box. In the world today, it has become almost commonplace for elected leaders to lock the door behind them once they achieve power. Vladi­mir Putin in Russia, Hugo Chávez in Venezuela, Yoweri Museveni in Uganda, Recep Tayyip Erdogan in Turkey — all found ways once in power to restrict opposition, muzzle the media and erode checks and balances.

Mr. Trump gives ample reason to fear that he would not respect traditional limits on executive authority. He promotes actions that would be illegal, such as torture. He intimates that he would use government to attack those who displease him. “They better be careful, they have a lot to hide,” he said last week of a wealthy Chicago family that had donated to a super PAC that opposes him. He promises diktats — impose a tariff, build a wall — as if Congress is nothing but an inconvenience. His vow to round up and deport 11 million undocumented immigrants, while perhaps not requiring congressional action, would necessitate a kind of intrusive police power this country has never seen. As Sen. Ted Cruz (R-Tex.) said before flip-flopping on the idea last week, such a roundup would require “jackboots to knock on your door and every door in America.”

Early in his campaign, conventional wisdom held that Mr. Trump could never win the Republican nomination. Now many experts assure us that he could never win a general election. Even if he did win, are not American institutions too resilient, and our system of checks and balances too deeply rooted, to be threatened by a populist demagogue like Mr. Trump? We hope so.

But even Mr. Trump’s campaign is an assault on democratic values. Politics is never free of rough-and-tumble accusations, wild exaggerations and unrealistic promises. But when Mr. Trump cheers the assault on one protester at his rally and says of another, “I’d like to punch him in the face,” that is something new and different. It is out of the ordinary when a candidate recycles demonstrably false Internet rumors — that thousands of American Muslims celebrated the attacks of 9/11, for example — and stands by them even when they are disproved. That he has difficulty repudiating the most odious white terrorist group in U.S. history; that he feels no need to explain, debate or defend his pie-in-the-sky proposals; that he degrades and disparages women, Mexicans, Jews, Muslims, people with disabilities — these are challenges to the core functioning of any democracy.

We would ask Mr. Trump’s would-be supporters to think carefully about where he is leading the nation before they vote in the primaries in Virginia and 11 other states Tuesday. There is nothing intrinsically racist in worrying that undocumented immigrants are taking jobs from U.S. citizens. It is not hateful to wonder whether terrorists might smuggle themselves into this country disguised as Syrian refugees. Plenty of patriotic Americans are concerned about the country’s direction and are disgusted by phony, consultant-driven politicians.

But Mr. Trump is pandering to those fears, not offering solutions. In so doing, he is insulting voters with genuine concerns. We continue to believe that Americans deserve better than that — and are better than that.