It all began with an unexpected proposal from a neighbor, who offered to put us in touch with his mentor, Harris Wofford. In writing Nine Days: The Race to Save Martin Luther King Jr.’s Life and Win the 1960 Election, we were straddling the line between past and present. Many of our subjects had long since died. But Wofford, then 89, provided the living link between Martin Luther King Jr. and John F. Kennedy.

As became evident during our long conversations in his Washington, D.C., condo overlooking Rock Creek Park, Wofford treasured this historical tie. He welcomed us with a patient willingness to tell, in great detail, just what living through the fraught and dangerous days of the Kennedy-Nixon election race felt like, and how those moments changed our nation’s course.

John F. Kennedy with his civil-rights adviser Harris Wofford on the South Lawn of the White House.

In 1960, Wofford, who went on to serve as a Democratic U.S. senator for Pennsylvania, had been part of a trio of civil-rights advisers on Kennedy’s campaign team. Alongside the Black journalist Louis Martin and Kennedy’s brother-in-law Sargent Shriver, Wofford disobeyed orders from above in agitating for the release of King, who had been jailed on a flimsy pretext after participating in an Atlanta Student Movement sit-in.

Harris Wofford provided the living link between Martin Luther King Jr. and John F. Kennedy.

Kennedy and his brother Robert initially saw intervening as an unacceptable risk; polls forecast a tied Black vote, and they worried that acting on behalf of King would cost them the white South that Nixon aimed to win. But the risk paid off, resulting in a crucial swing of Black voters toward the Democratic Party, not only in the 1960 election but beyond, to our own time. (Today, roughly 83 percent of Black voters identify with the Democratic Party or lean Democratic, compared with just 10 percent who identify with the Republican Party or lean Republican.)

Louis Martin, known as “the godfather of Black politics,” with Lyndon B. Johnson in the White House colonnade.

Wofford also encouraged us to dive into the papers of Louis Martin, whose unpublished recollections revealed details behind the trio’s support of King. We loved getting to know Martin’s daughters, and we gained a sense of just how crucial a role their father, a savvy strategist referred to by many as “the godfather of Black politics,” played in the civil-rights era—and just how often he’d been neglected in histories of midcentury American politics.

The story got richer as we built relationships with surviving Atlanta Student Movement veterans, including its leader, Lonnie King (no relation to M.L.K.). Like Harris, Lonnie kept us laughing and learning as he spoke about racial justice in the Georgia of decades ago. We learned how he recruited Morehouse, Spelman, and other Atlanta college students into the civil-rights movement, and how the students’ willingness to be arrested drew M.L.K. into a role he had resisted up until October of 1960: that of facing down imprisonment.

King under arrest, 1960. Atlanta Student Movement leader Lonnie King is beside him.

Lonnie King helped us understand a young M.L.K., new to most of us. As a neighbor and friend of the King family, it was Lonnie who pressured the 31-year-old minister, who had just moved home to Atlanta, to join a sit-in with the students. M.L.K.’s brave decision paved the way for the national leadership we remember in places such as Birmingham and Selma.

Thanks to the recollections of Wofford and Lonnie King, as well as to the recent unsealing of the papers of M.L.K.’s brilliant lawyer Donald Hollowell, we were able to write the first full account of the dramatic final weeks of the 1960 election. It was in Hollowell’s papers that we found the transcript of M.L.K.’s hearing, where King was sentenced to four months in the Georgia State Prison at Reidsville. The dramatic story of how his sentence was overturned is a far more complex and important one than merely Kennedy’s sympathetic call to Coretta King.

Heartbreakingly, both Harris and Lonnie King passed away before we could put the book into their hands. But they left examples of courage we can all learn from.

Stephen Kendrick and Paul Kendrick, father and son, have co-authored numerous history books, including Sarah’s Long Walk and Douglass and Lincoln

Their latest book, Nine Days: The Race to Save Martin Luther King’s Life and Win the 1960 Election, is out now from Farrar, Straus and Giroux