One of the oldest imperatives of American electoral politics is to define your opponents before they can define themselves. So it was not surprising when, in the summer of 1963, Nelson Rockefeller, a centrist Republican governor from New York, launched a preëmptive attack against Barry Goldwater, a right-wing Arizona senator, as both men were preparing to run for the Presidential nomination of the Republican Party. But the nature of Rockefeller’s attack was noteworthy. If the G.O.P. embraced Goldwater, an opponent of civil-rights legislation, Rockefeller suggested that it would be pursuing a “program based on racism and sectionalism.” Such a turn toward the elements that Rockefeller saw as “fantastically short-sighted” would be potentially destructive to a party that had held the White House for eight years, owing to the popularity of Dwight Eisenhower, but had been languishing in the minority in Congress for the better part of three decades. Some moderates in the Republican Party thought that Rockefeller was overstating the threat, but he was hardly alone in his concern. Richard Nixon, the former Vice-President, who had received substantial Black support in his 1960 Presidential bid, against John F. Kennedy, told a reporter for Ebony that “if Goldwater wins his fight, our party would eventually become the first major all-white political party.” The Chicago Defender, the premier Black newspaper of the era, concurred, stating bluntly that the G.O.P. was en route to becoming a “white man’s party.”

But, for all the anxiety among Republican leaders, Goldwater prevailed, securing the nomination at the Party’s convention, in San Francisco. In his speech to the delegates, he made no pretense of his ideological intent. “Extremism in the defense of liberty is no vice,” he said. “Moderation in the pursuit of justice is no virtue.” (He delivered that famous line shortly after the delegates had defeated a platform plank on civil rights.) Goldwater’s crusade failed in November of 1964, when the incumbent, Lyndon Johnson, who had become President a year earlier, after Kennedy’s assassination, won in a landslide: four hundred and eighty-six to fifty-two votes in the Electoral College. Nevertheless, Goldwater’s ascent was a harbinger of the future shape of the Republican Party. He represented an emerging nexus between white conservatives in the West and in the South, where five states voted for him over Johnson.

The reason for the shift was clear. Many white Southern Democrats felt betrayed by Johnson’s support of civil rights. The civil-rights movement had learned how to translate grassroots activism into political power. Among government leaders, L.B.J. was singularly important to the passage of the Civil Rights Act of 1964, and he stood firmly behind the Voting Rights Act of 1965. In both cases, he pressed on white Southern Democrats in Congress who had long supported the racist culture and strictures of Jim Crow. Until the mid-twentieth century, it was the Republican Party, founded a century earlier by Northerners enraged by the expansion of slavery—the “party of Lincoln”—that looked more favorably upon the rights of Black Americans. In 1957, it was a Republican President, Eisenhower, who deployed troops to intervene on behalf of Black students in the school-integration crisis in Little Rock. Goldwater’s rise proved the catalyst for change. As the historian Ira Katznelson told me, Goldwater opposed the Civil Rights Act mainly for libertarian reasons: “Nonetheless, it was a signal, and opened up possibilities for a major realignment.”

Establishment leaders of the G.O.P. were concerned that Goldwater had opened up the Party, which had barely emerged from the shadow of McCarthyism, to fringe groups on the far right, such as the John Birch Society—people whom Nixon referred to as “kooks.” (Robert H. W. Welch, Jr., the founder of the society, claimed that the goal of the civil-rights movement was to create a “Soviet Negro Republic.”) Marsha Barrett, a historian at the University of Illinois Urbana-Champaign, who chronicles the evolving relationship between civil rights and the Republican Party in her forthcoming book, “The Politics of Moderation: Nelson Rockefeller’s Failed Fight to Save the Party of Lincoln,” notes that, before Rockefeller issued his broadside, George W. Lee, a Black civil-rights activist, businessman, and lifelong Republican, wrote to Robert Taft, Jr., the Ohio Republican who ran for Congress in 1962. Failing a significant intervention, Lee said, “the Republican Party will be taken over lock, stock, and barrel by the Ku Kluxers, the John Birchers and other extreme rightwing reactionaries.”

Yet, once it became clear that Goldwater could win the nomination, shock at his extremism on a number of issues, including the potential use of nuclear weapons, began to morph into compliance. Taft’s behavior was typical of the trend. Although his family had long been a mainstay of the Republican Party—his grandfather had been President; his father, a senator—he endorsed Goldwater. Barrett told me that Goldwater’s rise was facilitated by the fact that “some moderate Republicans were simply trying to protect their own political prospects.”

In the contemporary Republican Party, the resonance is obvious.