The tension between big-tent inclusiveness and ideological purity has bedevilled our two major political parties for many years, but for Democrats it became especially vexing in the middle decades of the twentieth century. From 1932 to 1964, the Democratic Party won seven out of nine Presidential elections and enjoyed an almost continuous majority in the House and the Senate. But who, exactly, was winning and what did victory mean? The answer was clear in only two intervals. The first was the initial phase of the New Deal, when Franklin Roosevelt’s economic-rescue proposals were swiftly passed into law by Congress and embraced by a nation traumatized by the Great Depression. The second came during the three-year period after the assassination of John F. Kennedy, when Lyndon Johnson and Congress went on a legislative spree that ended with the midterm election in November, 1966.

“The Fierce Urgency of Now” (Penguin Press), Julian E. Zelizer’s account of wins and losses in the Johnson years, combines history with political science, as befits our data-happy moment. The information comes at us steadily—there are useful facts on almost every page—but the narrative is spartanly furnished. There’s little portraiture, not much drama, and only enough mood-setting context to let us know what America was up to while L.B.J. and Congress were contriving new ways to strengthen the social safety net and exhaust the national treasury. The emphasis falls instead on the high, and sometimes low, workings of legislative government, as bills inched through committees and subcommittees, nicked and scarred in “mark-up” sessions; the feint-and-parry of parliamentary maneuver; and, above all, the votes.

This patient no-frills approach offers illuminations that a more cinematic treatment might not. And if Zelizer, a professor of history and public affairs at Princeton, at times betrays the head-counting instincts of a House whip, well, head-counting is the nuts and bolts of congressional lawmaking, as scholars like Nelson Polsby and David Mayhew pointed out a generation ago, and as Ira Katznelson, Sarah Binder, and Frances Lee have done more recently. “Overshadowed by presidents and social movements, legislators remain ghosts in America’s historical imagination,” Zelizer observed in “The American Congress,” the large and very useful anthology he edited in 2004. Its analyses, by him and thirty-nine others, begin with the Continental Congress and go all the way up to the Clinton and Bush years—not likely to be known as the Gingrich or DeLay years, even as these scholars cut the Leaders of the Free World down to their proper constitutional size.

The idea of an imperial Presidency was always an exaggeration. “A President, these days, is an invaluable clerk,” Richard Neustadt, the dean of Presidential theorists, pointed out in 1960.The clerk at the time was Dwight Eisenhower, the general and war hero twice elected in landslides, only to be frustrated, like so many popular Presidents before and since, in skirmishes with well-organized adversaries on Capitol Hill. Congressmen were the heavies then, just as they are today. And yet, however much we say that we dislike our representatives, we keep sending many of them back to Washington. Together, Mitch McConnell and Harry Reid, each his party’s leader in the Senate, have spent fifty-eight years there. In the House, John Boehner and Nancy Pelosi have logged a combined fifty-two. These four, and some others, compose our democracy’s only long-term elected class.

What distinguished L.B.J. from almost all his predecessors and successors was his profound rootedness in Congress, where he spent a dozen years in the House and another dozen in the Senate. As Majority Leader, he became as famous as a senator could be, thanks to his resourcefulness and his genius for compromise and his almost feral magnetism. But it was seldom clear what L.B.J. really wanted, apart from dominating the game and intimidating the other players. Robert Caro has turned the question over on a spit in four immensely detailed volumes and still seems undecided. Real-time observers were mystified, too. “The test will come when he runs out of ideas,” Arthur Schlesinger, Jr., remarked in August, 1964. “Up to this point he has been living intellectually on the Kennedy years.” A year later, L.B.J. had signed Medicare and the Voting Rights Act into law, seven days apart.

But in politics the truly new idea is a rarity. Much of the agenda presented with such fanfare in the nineteen-sixties, first by Kennedy and then by Johnson, had been in congressional circulation since the nineteen-forties and fifties, and Johnson was well versed in its fine points. As early as 1949, Senator Hubert Humphrey, who had a master’s degree in political science, was proposing one bill after another on national health insurance, Social Security extensions, and federally financed school construction. Not one had a chance of becoming law then, because the votes weren’t there. They’d gone missing in F.D.R.’s second term, when an alliance of Republicans and Southern Democrats formed “the conservative coalition,” a bloc that functioned as an autonomous congressional party, supplanting the two nominally major ones. By the time L.B.J. became President, Congress had been, as Zelizer says, a “graveyard of liberal legislation” for more than a quarter century.

As late as 1960, some thought a vigorous new President might single-handedly revitalize Congress. This was a delusion. In August, a few weeks after Kennedy promised a “New Frontier” in his acceptance speech at the Democratic Convention, the Times’ Tom Wicker cautioned that if Kennedy won he would meet the fearsome “roadblock in Congress,” the House Committee on Rules, which, under the control of a “six-man conservative junta,” was “virtually a third branch of Congress—equal to and sometimes superior to the Senate and the House.” It was controlled by its chairman, Howard Smith, of Virginia, who decided which bills reached the floor for a general vote and which did not.

In the Senate, Southern domination was even greater. Johnson had spent his years as Majority Leader in continual negotiation with the chamber’s true “master,” Richard Russell, Jr., the Georgia segregationist whose weapon was the filibuster—not the lone-wolf stunts performed recently by Rand Paul and Ted Cruz but martial epics, involving a “platoon” of men who delivered four-hour monologues on a rotating basis. “The Senate might be described without too much violence to fact as the South’s unending revenge upon the North for Gettysburg,” the journalist William S. White wrote in “Citadel,” a best-selling account of the Senate published in 1957. White, a Texan, was by no means a critic of the Senate. He was enamored of its rituals and thought it had a “touch of authentic genius.” At a lunch organized by Johnson, not long after the book was published, each freshman senator found a copy of “Citadel” at his place setting with two inscriptions, one by White, the other by L.B.J., who urged them all to study it “as a sort of McGuffey’s Reader,” one of the freshman, Joseph Clark, recalled.

Clark, who called Congress “the sapless branch,” belonged to the growing and restive corps of liberal Democrats who found the Senate less the genteel club that White described than a mildewed establishment. Part of the problem was bipartisanship. “L.B.J. has no idea of his own but consensus,” Schlesinger noted. The criticism rings strangely today, when consensus and bipartisanship have become the holy grails of government, but in the mid-twentieth century they seemed symptoms of stasis and even atrophy. Humphrey, who was in the vanguard of a fresh style of Democratic politics—“issues-based” rather than interest-group-appeasing—inveighed against the “rotten political bargain” that Southern Democrats, including Johnson, had sealed with Republicans. Genuine political progress, he maintained, had to begin with institutional reform.