Robert Caro has been tracking his great white whale for thirty years now. As with any undertaking of this scale, an aura of legend attaches to the labor. First there is the Ahab-like devotion with which he has pursued the life of Lyndon Baines Johnson. In 1977, not long after publishing his epic biography of Robert Moses, New York City’s master builder, Caro decamped to Texas Hill Country for three years to take in the air of LBJ’s childhood. He spent a night outdoors in a sleeping bag to better fathom the desolation of the territory. Along with his wife, Ina, he has combed through every possible archive and ballot box; his appetite for firsthand impressions from LBJ’s entourage is matched only by his allergy to post-1960 scholarship. All of this fact-hunting and what you might call Method research has made Caro—who started his career as a reporter for Newsday—something of a hero for American journalists: he is the guildsman who made good and raised their craft to a level that academics can only envy. But he is far from universally admired by historians. Garry Wills and Sean Wilentz have dismissed him as a myth maker who rhapsodizes the life of Johnson into a morality play. In their view, Caro is hopelessly committed to seeing the thirty-sixth president through the prism of good and evil: LBJ the civil rights crusader versus LBJ the scourge of Vietnam. Caro’s anatomy of political power is too crude, they argue; he thinks LBJ’s secret was simply to be always the greediest, most ambitious and ruthless man in the room.

This is a serious criticism, but like the journalistic halo over Caro, it confuses the trappings of his achievement for its core. Caro has always been more valuable as a guide to how power works in postwar America in particular than how it works in some general abstract sense. Biography would not initially seem to be the form best suited to his purpose. The locus of power in this country is never fixed; it doesn’t reside in one person or single power elite, or in one institution, agency, economic interest, media outlet or popular movement, but in the shifting imbalances among them. Caro’s fortune in choosing LBJ as an entry for understanding the elements of American power is that Johnson moved through so many of them—and responded to and manipulated so many more—throughout his long career. Indeed, the great drama of reading The Years of Lyndon Johnson comes in watching LBJ master the machinery of American politics like one of those security contractors hired by companies to test the strengths and weaknesses of their systems....