Protesters in Washington, D.C., on Thursday. (Alex Wong/Getty Images)

Since Thomas Jefferson’s concubine, Warren Harding’s love nest and Bill Clinton’s innovative intern program, Americans have debated the role of character in leadership. But the concept of character has often been defined too narrowly. Sexual ethics — involving a range of behaviors from doomed longing to cruel exploitation — is a part of it, but not the largest part. “The sins of the flesh are bad,” wrote C.S. Lewis, “but they are the least bad of all sins. All the worst pleasures are purely spiritual: the pleasure of putting other people in the wrong, of bossing and patronizing and spoiling sport, and back-biting; the pleasures of power, of hatred.”

Republicans are beginning to see that the main problem with their presumptive nominee is not his lack of basic knowledge or his inability to stay on the script of sanity for 10 minutes at a time. The problem is Donald Trump’s public character, which no amount of last-minute coaching can change.

Trump’s instincts were on full display in his reaction to the Orlando terrorist attack. There was a pronounced lack of empathy for victims. There was a resort to insanely partisan conspiracy theories — including insinuations that President Obama is the Manchurian Muslim. There was an almost gleeful credit grab in asserting that his accusations about the violent nature of Islam were vindicated.

Trump’s resulting pronouncements — doubling down on immigration restrictions and raising questions about the loyalty of American Muslims — are counterproductive to the task of counterterrorism, undermining domestic cooperation on homeland security and complicating relations with allies and proxies. This is just terrible policy. But it is Trump’s moral worldview that results in terrible policy and promises worse to come. He believes that events always vindicate his instincts, which involve racial, religious and ethnic prejudice. He believes that any political tactic — including accusing your opponents of being enemy agents (as Joe McCarthy did) — is justified to further his interests. And he holds a Putin-like conception of how a great power should behave.

Trump’s worldview offers no limiting principles when it comes to the use and abuse of power. He is not an institutionalist — the kind of politician who venerates our constitutional system and its balances. He is not a tea party constitutionalist — the kind of politician who holds an ideological commitment to limited government and is suspicious of executive power. He is not a civil libertarian — the kind of politician concerned about the rights of individuals and groups.

Here are five GOP lawmakers who have taken issue with the ways their party's presidential candidate has reacted to the massacre in Orlando. (Sarah Parnass/The Washington Post)

The presumptive Republican nominee has already proposed the largest police operation (by far) in American history — the rounding up of more than 11 million people and forcing them across the border. What limiting principle would prevent a roundup of all Muslims? Trump has already proposed the murder of terrorists’ families. What is the limiting principle that would prevent his use of nuclear weapons against the Islamic State capital of Raqqa? Trump has already raised the possibility that Obama is a Kenyan and a jihadist and that Hillary Clinton was involved in Vince Foster’s murder. What limiting principle would prevent President Trump from targeting congressional opponents with innuendo that they are traitors or murderers, or any other accusation that Alex Jones puts on the Web? Trump has already proposed changing libel laws in order to restrict media criticism against him. What limiting principle would prevent him from, well, changing libel laws to restrict media criticism against him?

None of this is reductio ad absurdum. These are the natural implications of a worldview. Under the stress of events, it is clear that Trump’s organizing commitments are ethnic nationalism and a belief that the American government is too weak — too constrained by political correctness — in dealing with threats to American identity. He is riding the line between clownishness and fascism.

Republicans may go into the Cleveland convention with the worst case of buyer’s remorse in American political history. Their presumptive nominee may have historically high disapproval rates, little money in the bank, almost no organization on the ground and may trail a weak Democratic nominee by a large margin.

For now, Republicans such as Paul Ryan and Mitch McConnell are content to criticize the candidate they have endorsed. But a party convention is an up-or-down moment. Will they allow the balloons to drop on a leader with a broken moral compass? Or will they try to change the convention rules — perhaps to require a supermajority in picking a nominee — in an act of desperate resistance? Either way, Republicans are learning the hard way that character counts.

Read more from Michael Gerson’s archive, follow him on Twitter or subscribe to his updates on Facebook .

Read more here:

Greg Sargent: Even Republicans agree: Trump failed his first leadership test miserably

Jonathan Capehart: One year ago, Trump descended that escalator and took political discourse down with him

Eugene Robinson: Endorsing Trump will leave a mark

Jennifer Rubin: Getting Trump ‘on message’ won’t make him acceptable