Caro wonders. He notes that everyone was equally sure Kennedy would not choose Johnson in 1960. Kennedy had picked him in part because Johnson could help ensure support in Texas and other Southern states, but by the fatal visit to Dallas in November 1963, Johnson's influence had fallen enough that Kennedy made some key decisions about the trip in a meeting to which Johnson was not invited. At the same time, an investigation into the finances of Johnson aide Bobby Baker was leading to questions about Johnson himself. Life magazine was planning a long investigation. Congressional hearings had started the morning of Nov. 22.
Given Johnson's lowered standing in Texas and the testimony in Washington, "the president's assurances that he would be on the ticket might start to have a hollow ring indeed," Caro writes.
Caro also questions a narrative dear to Kennedy admirers.
At the time of JFK's assassination, a coalition of Republicans and Southern Democrats were blocking his legislative agenda, including a tax cut and a civil rights bill. Johnson got them passed, along with Medicare, education and other initiatives that Kennedy couldn't get through.
Former Kennedy aides Ted Sorensen and Arthur M. Schlesinger Jr. are among those who have said that those bills also would have gotten through had JFK lived. They reasoned that Kennedy, who won narrowly in 1960, would have been re-elected by a substantial margin and enjoyed larger majorities in Congress.
But Caro points out that the Senate committees were dominated by experienced and conservative Southerners with their own agenda. An especially hard case was Sen. Harry Byrd of Virginia, the deficit hawk and chair of the Senate Finance Committee. He had refused to act on the tax cut bill when Kennedy was in office, but, thanks to LBJ's charm and flattery, eventually cleared it.
"There had been times before (in the 1930s) when (Franklin) Roosevelt had huge majorities in Congress, but after the Southern Democrats decide no more New Deal legislation is going through, no more New Deal legislation goes through," Caro says, adding that obstruction lasted into the 1950s, until Johnson became majority leader.
"Only one guy got bills through — it was Lyndon Johnson."
Caro was friendly with Sorensen, Kennedy's devoted speech writer who died in 2010. They were neighbors on Manhattan's Upper West Side and would meet often, the two sitting on opposite couches in Sorensen's apartment, overlooking Central Park. Sometimes, they would discuss whether Kennedy could have gotten his legislation passed.
"I remember going over this again and again with Sorensen," Caro says. "He wouldn't agree with me."
The historian says the book tells two stories: "The deep hatred" between Johnson and the Kennedys, especially Robert Kennedy, and what happens when JFK is dead and roles are overturned. Johnson, the unwanted vice president, is in charge.
"That's why I call the book 'Passage of Power.' The title is what it is. You examine something in its moment of greatest crisis and you see what it has to do," Caro says. "To watch Lyndon Johnson grab up the reins of power and get Kennedy's legislation moving, how he keeps the people in the Kennedy administration from leaving and reassures the American people, is to see political genius in action."
Caro has called Johnson a story of darkness and light, and clouds will gather in Volume V, which Caro expects to complete within the next few years. Among what happened during Johnson's last decade: His landslide victory in 1964; his fateful decision in 1965 to commit ground troops to Vietnam; the rapid passage of historic bills, including on civil rights, education and immigration; Robert Kennedy's brief run against Johnson for president in 1968; the assassinations of Kennedy and Martin Luther King Jr.; Johnson's announcement that he would not seek another term; his final years back in Texas and his death, in 1973.
The book, Caro says, will be long.