For George W. Bush, the summer already looks unbearable. The party he gave his life to will repudiate him by nominating a bombastic serial insulter who makes the famously brash former president look like a museum docent by comparison. And a renowned presidential biographer is weighing in with a judgment that makes Mr. Bush’s gentleman’s Cs at Yale look like the honor roll.

If Mr. Bush eventually gets a more sympathetic hearing by history, as he hopes, it will not start with Jean Edward Smith’s “Bush,” a comprehensive and compelling narrative punctuated by searing verdicts of all the places where the author thinks the 43rd president went off track. Mr. Smith’s indictment does not track Donald J. Trump’s, but the cumulative effect is to leave Mr. Bush with few defenders in this season of his discontent.

Mr. Smith, a longtime academic and finalist for the Pulitzer Prize, made a name for himself in part with masterly biographies of Dwight D. Eisenhower and Ulysses S. Grant, offering historical reassessments of underrated presidents who looked better with the passage of time. With “Bush” he sticks to the original conventional assessment, presenting a shoot-from-the-hip Texan driven by religiosity and immune to the advice of people who knew what they were talking about.

While not a fresh portrait, it is one worth debating at a time when the political class is struggling to understand the meaning of Mr. Trump’s rise. Mr. Trump’s name appears nowhere in “Bush,” but it is clear the populist revolt that propelled him to the verge of the Republican nomination had its roots in Mr. Bush’s presidency, so much so that he easily overcame the former president’s brother Jeb. Mr. Trump rejects much of what George W. Bush stood for, from the war in Iraq and more forgiving immigration policies to free trade and the very notion of compassionate conservatism.