The easy part, the transition to power, is over. The hard part begins now. So this concern arises: When President Trump is buffeted by events — when hard times hit, when crises arise, when other politicians and world leaders do not bend to his will — pernicious things will happen. Rather than try to address the alienation and anger that exists in America, he will amplify them. He’ll create yet more conspiracy theories. He will also go in search of enemies — the press, the opposition party, other nations, even Republican leaders — in order to create diversions that inflame his most loyal supporters. And when he locates his targets, he will do what is second nature to him, which is to try to delegitimize and destroy them. What’s different now is that he will have the additional, awesome power of the presidency at his disposal.

We cannot know the outcome of events in advance, but we can draw reasonable inferences. One of them is this: In failing to distinguish between the good of the nation and his own vanity, the danger is that Mr. Trump will fail to see the limits of his authority and will try to use both the bully pulpit and the power of government — the I.R.S., the F.B.I., regulatory agencies and others — to settle personal scores. He’ll do what he needs to in order to get his way. That has been the animating force in his life.

In “The Abolition of Man,” C. S. Lewis wrote, “I am very doubtful whether history shows us one example of a man who, having stepped outside traditional morality and attained power, has used that power benevolently.”

Donald Trump has not only spent much of his life stepping outside of traditional morality; he seems to delight in doing so. If I am right about Mr. Trump, and Lewis is right about history, then it is unlikely that President Trump will use his power benevolently. Quite the opposite, in fact.

Because Republicans control Congress, they have the unique ability and the institutional responsibility to confront President Trump.

What this means is that Republican leaders in Congress need to be ready to call Mr. Trump on his abuses and excesses, now that he is actually in office. It is a variation of the Golden Rule, in this case treating others, including a Republican president, as they deserve to be treated. They need to ask themselves a simple, searching question: “If Barack Obama did this very thing, what would I be saying and doing now?” — and then say and do it.

In anticipating a Trump presidency, I wish my hopes exceeded my fears. But Donald Trump has given us many reasons to worry. A man with illiberal tendencies, a volatile personality and no internal checks is now president. This isn’t going to end well.