That sound you hear is the wall of elected Republican support for President Trump beginning to crack.

There was Sen. Lindsey O. Graham (S.C.) warning that firing special counsel Robert S. Mueller III would be “the beginning of the end of the Trump presidency.” There was Sen. Charles E. Grassley (Iowa) warning that if Attorney General Jeff Sessions were fired there is “no way” that the Judiciary Committee would consider a replacement this year. And there was Sen. Ben Sasse (Neb.) warning Trump that if he were considering a recess appointment to replace Sessions, he should “forget about it.” Added Sasse: “The presidency isn’t a bull, and this country isn’t a china shop.”

All of these responses to future abuses of power are important as preventive measures. But no major elected Republican has provided a comprehensive critique of Trumpism itself. Until now. Sen. Jeff Flake’s new book, “Conscience of a Conservative,” is a white-hot indictment of Republican cowardice in the face of a hostile ideological takeover. It also represents the single largest act of political bravery of the Trump era.

The book has gained buzz for its, well, forthright description of Trump himself. He is guilty of “erratic behavior, unmoored from principle,” and a “fly off the handle” approach to governance. He is “impulsive and lacking in coherent economic analysis.” He is willing to “heap praise on dictators and to speak fondly of countries that crush dissent and murder political opponents” and to “undermine confidence in our democratic elections.”

As an author, Flake has the ability to employ language like a legal but damaging body blow. “Seemingly overnight,” the Arizonan writes, “reckless, outrageous, and undignified behavior was excused and countenanced as ‘telling it like it is,’ when it was actually just reckless, outrageous and undignified.” And: “We degrade our politics enough as it is without turning our democracy over to carnival barkers and reality television.”

I would pay to witness the next meeting between the Senate Republican caucus and Trump, just to see the interaction between Flake and the barker in chief.

Flake’s ideological critique is libertarian in content (Friedrich Hayek and Barry Goldwater figure prominently) but moderate in application. He defends free trade and views globalization as a fact and an opportunity. But he actually apologizes for opposing the TARP bank bailout in 2008, a vote that he self-diagnoses as an “act of cowardice.”

The passion of “Conscience of a Conservative,” however, comes from Flake’s deep, religiously rooted outrage at “the dehumanization of vast groups of people based on nationality or ethnicity.” This is what led him to visit a mosque in a show of solidarity when candidate Trump proposed a Muslim travel ban. This is what led Flake, during the first meeting of Senate Republicans with Trump after the election, to question Trump about his characterization of Mexican immigrants as “rapists.”

Flake’s view of Mexican migrants was conditioned by their work — hard, reliable work — on his family ranch in Arizona. “I have always said that I could never look at these migrants and consider them criminals.” And the moral center of the book is the moving story of two doctors — one a Palestinian, the other an Afghan — who saved the life of Flake’s father after a ruptured aorta. “Neither might have been allowed into America,” says Flake, “had we restricted visas from countries compromised by terrorism.”

Flake is a member of the Church of Jesus Christ of Latter-day Saints, and one explanation for the skepticism many of his fellow Mormons share about Trump is surely their focus on personal character and rectitude — areas where the president is defiantly lacking. But another is their own history as victims of persecution. “Mormons,” explains Flake, “have had foundational and horrifying experience with some of these worst impulses of mankind and became both refugees and immigrants in our own land. And so when someone starts talking of religious tests and religious bans, we know better. . . . When we say ‘No Muslims’ or ‘No Mexicans,’ we may as well say ‘No Mormons.’ Because it is no different.”

History tends to honor those who are the first to speak with moral clarity in a muddy time. Read Hubert Humphrey’s speech on racial equality to the 1948 Democratic convention: “My friends, to those who say that we are rushing this issue of civil rights, I say to them we are 172 years late.”

It was admirable to be for equality in 1964. It was courageous and visionary in 1948. Flake’s party is different, but his role is comparable.

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