Moreover — quaint as this may seem today — he believed that principles were not things to be surrendered to polls and lobbyists and that clamorous mob called “the base.” On the wall of his district office in Piqua, Ohio, McCulloch displayed a framed excerpt from Edmund Burke’s message to the electors of Bristol: “Your representative owes you, not his industry only, but his judgment; and he betrays, instead of serving you, if he sacrifices it to your opinion.”

The Kennedy and Johnson administrations knew they would need a large contingent of Republicans to get the civil rights bill past the segregationist Southern Democrats who held the commanding heights on Capitol Hill. And so they sent an emissary to McCulloch, who was the senior Republican on the House Judiciary Committee, and enlisted him as a partner. He agreed to an active collaboration with the Democratic White House, an alliance hard to imagine today and even then viewed by some in his party as bordering on treason. He had two conditions. First, if McCulloch helped get a strong bill through the House, he insisted the president would not allow it to be weakened in the Senate, where the oligarchy of Southern Democrats had successfully filibustered past civil rights measures until they were rendered toothless. Second, McCulloch wanted assurances that Republicans would share the credit for passage.

McCulloch’s story is rescued from obscurity in Todd Purdum’s forthcoming account of the great battle, “An Idea Whose Time Has Come: Two Presidents, Two Parties, and the Battle for the Civil Rights Act of 1964.” Purdum, a former New York Times colleague who now writes for Vanity Fair and Politico, has composed a suspenseful legislative procedural that is a synthesis of history and fresh reporting. His account of the Civil Rights Act is rich in characters, including other Republicans instrumental in passing the landmark bill, like Senator Everett Dirksen of Illinois and Representative Charles Halleck of Indiana, the Republican leader in the House. (Those were the days when “party of Lincoln” actually meant something.) But Purdum has a particular affection for Bill McCulloch, who became, as one legislative aide put it, “the conscience of the bill.” McCulloch assured the bill was the toughest and most enforceable that could muster a majority, and he stiffened the spines of President Johnson and his attorney general, Robert Kennedy, when the opposition got tough. The Southerners’ filibuster dragged on for 57 days but was ended without major concessions, the first time supporters had ever broken a filibuster on a civil rights bill.

The final version outlawed discrimination in hotels, restaurants and other public accommodations. It empowered the attorney general to bring suit to desegregate public schools. It prohibited discrimination in hiring, and let victims of such discrimination seek redress in the courts. And it expanded protection of the right to vote, which would be greatly strengthened the following year. (Yes, McCulloch was a champion of the Voting Rights Act, too.) In the end, the Civil Rights Act passed with a larger percentage of Republicans in support than Democrats.

Among the documents found in McCulloch’s papers years after his death was a handwritten and unusually heartfelt letter from the former first lady, Jacqueline Kennedy, who wrote: “Your integrity under such pressures is what makes our political system worth fighting for and dying for. Please forgive the emotional tone of this letter — but I want you to know how much your example means to me. It is a light of hope in an often dark world, and one I shall raise my children on as they grow older.”