Donald Trump at a rally in Council Bluffs, Iowa, on Wednesday. (John Locher/Associated Press)

IF YOU know that Donald Trump is ignorant, unprepared and bigoted, but are thinking of voting for him anyway because you doubt he could do much harm — this editorial is for you.

Your support of the Republican presidential nominee may be motivated by dislike of the Democratic alternative, disgust with the Washington establishment or a desire to send a message in favor of change. You may not approve of everything Mr. Trump has had to say about nuclear weapons, torture or mass deportations, but you doubt he could implement anything too radical. Congress, the courts, the Constitution — these would keep Mr. Trump in check, you think.

Well, think again. A President Trump could, unilaterally, change this country to its core. By remaking U.S. relations with other nations, he could fundamentally reshape the world, too.

Of course, in many areas Mr. Trump would not have to act unilaterally. If he won, chances are Republicans would maintain control of Congress. GOP majorities there would be enthusiastic participants in much of what Mr. Trump would like to do: gutting environmental and workplace regulations, slashing taxes so that the debt skyrockets, appointing Supreme Court justices who oppose a woman’s right to have an abortion. In areas where Republican officeholders such as House Speaker Paul D. Ryan (Wis.) and Senate Majority Leader Mitch McConnell (Ky.) imagine themselves acting as a brake on Mr. Trump’s worst instincts, skepticism is in order. If these supposed leaders are too craven to oppose Mr. Trump as a candidate, knowing the danger he presents, why should we expect them to stand up to the bully once he was fully empowered?

But say they did — or imagine, also improbably, that Mr. Trump faced a Democratic Congress. The president would appoint officers — a budget director, an attorney general, a CIA chief — who were disposed to let him have his way. And in the U.S. system, the scope for executive action is, as we will lay out in a series of editorials next week, astonishingly broad. At times we have questioned President Obama’s sweeping use of those powers even when we agreed with his goals, such as his broad grant of amnesty to millions of undocumented immigrants. Mr. Trump could push it much further.

Could he tear up long-standing international agreements? Round up and expel millions of longtime U.S. residents? Impose giant tariffs? Waterboard terrorist suspects? Yes, yes, yes and yes — all without so much as an if-you-please to Congress. Could he bar the media from covering him? To a large extent, yes. Could he use the government to help his businesses and, as he has threatened, injure those he perceives as enemies? Yes, he could.

Given Mr. Trump’s ever-evolving positions, and the apparent absence of fundamental beliefs other than in his own brilliance, it would be foolish to make flat predictions of how he would behave. Nor do we underestimate the resilience of the U.S. system or the devotion that U.S. government workers bring to the rule of law.

But it would be reckless not to consider the damage Mr. Trump might wreak. Some of that damage would ensue more from who he is than what he does. His racism and disparagement of women could empower extremists who are now on the margins of American politics, while his lies and conspiracy theories could legitimize discourse that until now has been relegated to the fringe. But his scope for action should not be underestimated, either. In our upcoming editorials, we will examine some arenas where Mr. Trump has been relatively clear about his intentions — and where presidential powers are mighty. We hope you will read them before you vote.