Lady Bird Johnson became first lady aboard Air Force One while President John F. Kennedy’s blood was still drying on her predecessor’s pink Chanel suit.

“I realized shortly after,” she later wrote, that “nobody else would live through the next months in quite the way that I would and see the events unroll from this vantage point.”

So Johnson, in her lilting East Texas voice, began talking into a tape recorder. She kept talking — and talking and talking — into the machine nearly every day of Lyndon B. Johnson’s presidency, leaving behind a remarkable but almost completely overlooked audio diary of one of history’s most momentous presidencies.

Those recordings, made available in their entirety during the last several years at the LBJ Presidential Library, are the subject of a new podcast series and Lady Bird biography by Julia Sweig, a Washington author who in studying all 123 hours of tapes discovered a first lady whose role in the Johnson presidency was more consequential than previously known.

“The diary is a new tool in trying to understand Lady Bird Johnson, but also, of course, indirectly, Lyndon Johnson and his whole presidency,” said Mark Atwood Lawrence, a Johnson scholar and director of the LBJ Presidential Library. “She wasn’t just a passive first lady in the background.”

Indeed, the tapes and Sweig’s other research reveal a partnership in just about every facet of Johnson’s presidency, which Lady Bird called “our presidency.”

She’s with him when he calls network TV presidents right after taking office. She counsels and consoles him on the Vietnam War escalation, discussing how pleasing anti-communist hawks might help win support for civil rights legislation.

And in perhaps the podcast and book’s biggest revelation, Sweig unveils a seven-page memo that Lady Bird wrote to Johnson and agonized over in her audio diaries when he was considering not running for president after completing Kennedy’s term.

In the memo, she drafted a fake news release so Johnson could, as Sweig writes in her upcoming biography, digest the “potent emotional impact of taking such a radical step.”

On the recording, Lady Bird sighs.

“I hope he won’t use it,” she says. “That’s that!”

But contained in that memo is an idea he did use: running for president in 1964 but bowing out after one term. The move stunned the country, but not Lady Bird.

“This was very much a political partnership, and she was an active member of his administration,” Sweig said in an interview. The tapes, she added, reveal “her role in history, observing it and helping make it.”

Sweig’s podcast, “In Plain Sight: Lady Bird Johnson,” which ABC News begins airing Monday, opens with the first words Lady Bird spoke into her tape recorder, her soft voice both unforgettable and forgotten to time.

“Friday, November 22nd,” she says. “It all began so beautifully. After a drizzle in the morning, the sun came out bright and clear. We were going into Dallas.”

And then the shots.

“Mrs. Kennedy’s dress was stained with blood,” Lady Bird goes on. “That immaculate woman exquisitely dressed and caked in blood. I ask her if I couldn’t get somebody to come in to help her change. She says, ‘Oh no, that’s all right, perhaps later.’ And then, with some element of fierceness, she said, ‘I want them to see what they have done to Jack.’”

Lady Bird’s decision to record that moment in history and then keep talking was instinctual. She studied journalism and history in college. During the Kennedy-Johnson campaign in 1960 she was often seen with a small notebook taking notes during her husband’s conversations.

Lady Bird recorded most of her diary entries on a blue velvet sofa in a second-floor room at the White House that she used as a dressing room and office.

“In winter, I often recorded sitting on the sofa looking at the fire burning merrily in the little corner fireplace,” she later wrote. “And in the summer I reversed one of the chairs and talked into my machine while I looked out over Andrew Jackson’s magnolias to the Washington monument — my favorite view in all of Washington, often outlined against the drama of sunset.”

Lady Bird, who died in 2007 at the age of 94, devised a meticulous system to document each day’s events. Aides provided copies of her and Johnson’s calendars, as well as newspaper clippings, seating charts from White House events, and copies of speeches. She also relied on handwritten notes that she continued to take in small reporter-type notebooks.

“Gradually I, the most unmechanical of women, made friends with this little machine and learned how to thread it and change it, and hold it in abeyance while I thought of my next phrase,” she wrote.

In 1970, two years after she helped Johnson with his speech declaring he would not run for reelection, Lady Bird published a redacted version of her diaries representing a small fraction of the nearly 1.8 million words she spoke into her recorder.

A friend told Sweig about them a few years ago, and when the writer later visited the LBJ library in hopes of writing a book on women and power, she walked into an exhibit where a clip from the Nov. 22 diary entry was playing.

Sweig was utterly captivated. Realizing the audio started with a motion detector at the doorway, Sweig kept walking in and out, over and over, to continue listening.

“And that just began this long process of listening to all those tapes and discovering who was behind that voice,” said Sweig, whose book, “Lady Bird Johnson: Hiding in Plain Sight,” will be published March 16.

Sweig’s research took roughly five years to complete — about the same amount of time that Johnson and Lady Bird were in the White House.

“As I look back on those five years of turmoil and achievement, of triumph and pain,” Lady Bird later wrote, “I feel amazement that it happened to me, and gratitude that I had the opportunity to live them, and strongest of all — out of all the trips that I made and all the people that I met — a deep, roaring faith in and love for this country.”

Correction: An earlier version of this story gave the wrong figure for the number of hours recorded by Lady Bird Johnson. It is 123 hours.

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