Donald Trump has been the most persistently unpopular first-term president in the postwar era. Much of the nation is exhausted and embarrassed by his presidency, pining for normalcy, eager to change the channel. The president’s own internal polls show Mr. Trump trailing the former vice president, Joe Biden, not only in many battleground states Mr. Trump won in 2016, but in traditional Republican strongholds like Georgia.

But as we saw Tuesday night, during a huge, raucous rally in Orlando, Fla., Trump is viewed by his supporters almost as a demigod. One excited Trump supporter who was there told me he was overwhelmed by the unwavering support for Mr. Trump, driven by a sense that Mr. Trump has been deeply wronged — by the Mueller investigation, by the media and by what he described as “anti-Trump forces.” He also told me, based on conversations he had with others at the rally, that Mr. Trump’s supporters believe his era is “spiritually driven.” What he meant by that is that person after person reported that when it comes to Mr. Trump and the presidency, “God has chosen him and is protecting him.” It is the Children of Light against the Children of Darkness.

That certainly aligns with my sense of how Trump supporters see things. It’s not just that Mr. Trump is exceedingly popular among Republicans, with his approval rating this year hovering in the high 80s and low 90s. It is that he has won their undying loyalty and affection. As a Republican friend of mine put it to me recently, Mr. Trump is the general leading the army into battle against an enemy that needs to be vanquished for the good of the nation. When facing an existential threat, there is no room for public dissent. In Mr. Trump’s Republican Party, you are expected to treat him with reverence, submission and obeisance, or you will be treated as a traitor to king and cause. Just ask Jeff Flake, Bob Corker, Mark Sanford and Justin Amash.

It was unthinkable when Donald Trump rode down the escalator at Trump Tower four years ago to announce his improbable run for the presidency, but his imprint on the Republican Party is at least as large as that of Ronald Reagan’s at a comparable point in his presidency. The Republican Party has been transformed by Mr. Trump.

That’s true in some areas more than others. In the realm of policy, Mr. Trump has pursued a fairly traditional Republican agenda on judicial appointments, abortion, tax cuts, deregulation and military spending. What makes Mr. Trump transformative is the areas in which he is redefining the right.

Let’s review. Until Mr. Trump, the Republican Party was committed, at least philosophically, to free trade. It is now led by a man who is instinctively protectionist and refers to himself as “Tariff Man.” The pre-Trump Republican Party championed limited government and entitlement reform; today it shows no interest in either. It was once unthinkable that a Republican president would target private companies in order to settle personal scores. For Mr. Trump, this is routine.

Republicans flayed President Barack Obama for implementing the Deferred Action for Childhood Arrivals program despite lacking the constitutional and legal authority to do so. Yet Republicans in Congress overwhelmingly supported Mr. Trump’s emergency declaration to fund border wall construction despite its being a clear violation of the separation of powers.

Past Republican presidents were deeply committed to American global leadership, the Atlantic alliance, good relations with allies like Canada and publicly calling out brutal regimes like North Korea. No more. Today the Republican Party is led by a man who levels attacks on Canada even as he admits he “fell in love” after exchanging “beautiful letters” with the leader of North Korea, Kim Jong-un. Mr. Kim, it used to go without saying, rules what is arguably the most repressive government on earth; this is the man to whom Mr. Trump makes enormous concessions while getting almost nothing in return. And let’s not forget, however remote it might seem now, that the idea that a Republican president would side with the leader of Russia rather than his own intelligence agencies was once unimaginable. Under Mr. Trump, it happened. Mr. Trump has also turned a party that for decades was pro-immigration and friendly with Mexico — and in the case of Reagan, in favor of amnesty for undocumented workers and against putting up even a fence on the southern border — into one that is increasingly antagonistic toward immigrants and relentlessly hostile to Mexico, the current thaw notwithstanding.

Mr. Trump has flipped the Republican Party from outward looking to inward looking, from the champion of an open society into the cheerleader of a closed one, from optimism to pessimism. (It’s a long road to travel to get from “Morning in America” to “American carnage.”) A party that once claimed to abhor “identity politics” now relies on them as its closing argument in elections.

But as significant as these changes have been, the Trump transformation of the Republican Party has been even more decisive and far-reaching in other realms.

Republicans once fashioned themselves as members of the party of ethics, morality and law-and-order; today they fiercely defend a president who is essentially an unindicted co-conspirator for authorizing hush money payments to a porn star, who is a promiscuous liar, a man whom Robert Mueller could not clear of obstruction of justice and who just last week indicated he would eagerly listen to a foreign power that offered damaging information on his opponent during the upcoming president race. He even criticized the F.B.I. director he chose for saying that the agency would want to know about any foreign election meddling.

The most withering line of the year, so far, came from one of the Democratic candidates, Pete Buttigieg, who referred to Vice President Mike Pence, an outspoken evangelical Christian, as a “cheerleader for the porn star presidency.” Many of those who during the Bill Clinton presidency insisted character and personal integrity were essential qualities in political leaders have in the Trump era decided such matters are utterly unimportant. By their refusal to confront those flaws and failures in Mr. Trump, they are complicit in the debasement of American culture and politics. Many of Mr. Trump’s most vocal and prominent evangelical supporters, because of their rank hypocrisy, are doing more to damage Christian witness than the so-called New Atheists ever could.

Beyond that, in their ferocious defense of the president, Trump supporters are signaling that decency is a form of weakness, that cruelty is a welcome and highly effective political weapon and that the low road is the preferred road. At one point, Republicans were willing to tolerate Mr. Trump’s brutish tactics and reprehensible character as the price of party loyalty; today many of them seem to relish it. They see the dehumanization of others as a form of entertainment.

All of this has come at a crushing price, including driving away young people in huge numbers. The Trump ascendancy has made far too many Republicans increasingly contemptuous of serious intellectual and policy argument, indifferent to empirical truth and disdainful of governing. They prefer to turn politics into an ongoing freak show. But the greater price is the indelible stain all this places on the integrity of a party many of us once believed in, served in and took pride in.

Mr. Trump’s takeover of the Republican Party is complete. Healing and renewal can’t begin until the party rejects the malignancy of Trumpism and embraces the belief that politics is not only a necessary activity but a noble calling, an imperfect but essential way to advance justice. That day may yet come. Right now it feels light years away.

Peter Wehner a senior fellow at the Ethics and Public Policy Center, served in the previous three Republican administrations and is a contributing opinion writer, as well as the author of “The Death of Politics: How to Heal Our Frayed Republic After Trump.”