The history of attempts to halt the rise of an alarming candidate, or even of one who just doesn’t fit a party’s idea of itself, offers no encouragement to those out to stop Donald J. Trump’s march toward the Republican nomination. When you reach a certain point, no cabal of “elders” can rescue anything. That’s what the Democratic Party establishment realized in 1976, when it couldn’t stop Jimmy Carter. It was hard to accept that a one-term Georgia governor who, moreover, said odd things (despite a fairly strong record on civil rights, he once used the phrase “ethnic purity” in connection with preserving a neighborhood’s character) could lead the party of Franklin D. Roosevelt and John F. Kennedy. A so-called A.B.C. movement—anyone but Carter—grew in intensity after Carter won the important Pennsylvania primary that April. As R. W. Apple wrote in the Times, “the ominous shadow of Hubert H. Humphrey”—the former Vice-President, who had lost to Richard Nixon in 1968 and was thought to be eager to try for the nomination—fell across Carter’s path to victory. Carter, in any case, went on to defeat a number of now forgotten Democrats with plausible Presidential qualities, among them the Senators Birch Bayh and Henry (Scoop) Jackson, and the Congressman Morris Udall. Humphrey never jumped in.

In 1964, Republican Party leaders were terrified by the ascent of the conservative Arizona senator Barry Goldwater. In retrospect, Goldwater was not all that frightening, although he did make some worrisome statements, such as speculating about whether to use nuclear weapons to defoliate forests in Vietnam. (He soon took that back.) Nor was Goldwater entirely doctrinaire. In a cheerful memoir of that time, “Flying High,” William F. Buckley, Jr., the founding editor of National Review, recalled that, when Goldwater needed a “philosophy,” a Buckley brother-in-law, L. Brent Bozell, Jr., ghostwrote the senator’s book “The Conscience of a Conservative.”

The Republican establishment, which then included a major newspaper, the New York Herald Tribune, persuaded the Party’s most prestigious and beloved elder, the former President Dwight D. Eisenhower, to write a column in which he urged his party to nominate someone who shared none of Goldwater’s views. Ike didn’t mention Goldwater by name, nor did he mention Goldwater’s main rival, Governor Nelson Rockefeller, of New York, who was expected to win the crucial, upcoming California primary. When the unstoppable Goldwater won (helped by the birth, three days before the primary, of a Rockefeller son with his second wife, whom he had married soon after both divorced their previous spouses), panic set in. Eisenhower and others encouraged William Scranton, the moderate governor of Pennsylvania, to make a last-minute challenge. But the governor’s allies quickly deserted him, leaving Scranton humiliated and Goldwater’s boisterous supporters in charge.

In a better world, Goldwater would have faced the incumbent President Kennedy in 1964, in what surely would have been a spirited, even educational contest between two men who liked each other while disagreeing strongly. Instead, he ran against Lyndon B. Johnson, who took office after Kennedy was murdered, and the race became a sour contest in which Americans had to choose between Goldwater, portrayed as a trigger-happy dimwit, and Johnson, viewed by many voters with distrust and dislike. As Robert Caro wrote in the third volume of his splendid Johnson biography, L.B.J., before the assassination, was about to be drawn into deeply embarrassing scandals connected to his protégé Bobby Baker, a former Senate page who had become suspiciously rich while serving as the secretary to the Senate majority. The scandal didn’t affect Johnson’s margin of victory in the general election, which was so large that the foundational voting patterns of the two parties were altered, especially in the Republicans’ loss of the black vote. ...