On March 31, 1968, at 9:00 p.m., Lyndon B. Johnson sat behind the large wooden desk he had used since his days in the Senate and addressed the American people from the Oval Office.

His speech concerned the increasingly unpopular war in Vietnam, which had come to cripple his 52 months in the White House, focusing on "peace in Vietnam and Southeast Asia."The president spoke for 40 minutes, then came to a passage he had added at the last minute:

"With American sons in the fields far away, with America's future under challenge right here at home, with our hopes and the world's hopes for peace in the balance every day, I do not believe that I should devote an hour or a day of my time to any personal partisan causes or to any duties other than the awesome duties of this office ... the presidency of your country."

"Accordingly, I shall not seek, and will not accept, the nomination of my party for another term as your president."

The announcement sent shock waves across the world. Johnson, a paragon of power, was relinquishing the ultimate political prize for the betterment of the nation.

It came at the front end of what he called "the nightmare year," a divisive and turbulent year that had already endured the seizure of a United States naval ship and capture of 83 sailors by North Korea, the Tet Offensive that would turn the tide of the Vietnam War, the year that would later see the assassinations of Martin Luther King, Jr. and Robert F. Kennedy; urban unrest and upheaval; violent anti-war protests at the Democratic National Convention in Chicago; and the invasion of Czechoslovakia by Soviet troops to quash liberal reform.

A popular notion, attributed without proof to Mark Twain, asserts, "History doesn't repeat itself but it rhymes." To be sure, we hear rhymes of 1968 in 2024, as we find ourselves caught up in the rapid currents of history flowing toward an uncertain and perhaps unsettling outcome—the dominance of Donald Trump within the GOP despite the disgraceful stain of Jan. 6, 2021, and his recent conviction on 34 felony counts; a politicized Supreme Court voting to grant broad presidential immunity expanding the office of the presidency and offsetting the balance of power; a disastrous presidential debate performance sealing the political fate of President Joe Biden; and the assassination attempt on Trump's life.

More analogously, we heard it in Biden's announcement on social media on Sunday stating that he will decline to accept the Democratic Party's presidential nomination next month in which he stated, "It has been the greatest honor of my life to serve as your president. And while it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as president for the remainder of my term."

The similarities between our 36th and 46th presidents abound. Both became creatures of Washington, D.C., gaining election to Congress at age 29 -- LBJ to the House, Biden to the Senate. Both thrived and developed a keen understanding of the nuances of power in the Senate, where LBJ ascended after 11 years in the lower chamber. Both became vice presidents for charismatic, elegant presidents who elicited inspiration and hope and cast long shadows. And both earned the presidency in their own right, forging consequential tenures around outsize legislative advances.

Now, of course, another common denominator will be that both boldly stepped aside—perhaps against their instincts to cling to the presidency—for the good of the country and their party.

It also bears mentioning that both did so for the betterment of their legacies. Had LBJ remained in power, he would have created further divisions and bound himself more inexorably to the Vietnam War. As Lady Bird Johnson said later, "I saluted him for being clearheaded enough to see that he wasn't the man at that particular juncture of time to unite the country." Moreover, Vietnam would have threatened to diminish the mammoth accomplishments of his Great Society, especially those around civil rights, his most enduring legislative triumphs. While it would take over two generations for the dark cloud of Vietnam to recede as passions around the war cooled over time, LBJ today is generally remembered more for his leadership on civil rights than as commander-in-chief of a failed war.

The main pillar of Biden's legacy will be the protection and preservation of democracy, and the revitalization and strengthening of America's alliances around the world, which he included in the list of presidential accomplishments he enumerated in his letter. Biden achieved those ends by wresting the White House from Trump, returning the nation to democratic values and norms, and reviving a weakened NATO around Russia's invasion of Ukraine. But he risked the undoing of those attainments with another Trump presidency as he played an increasingly weak hand in his reelection bid. By folding and advocating the candidacy of Kamala Harris, his vice president and heir apparent, who stands a greater chance of staving off another Trump Administration, he better ensures the protection of his most important contribution to the nation and the world—and ultimately his principal legacy.

Next Monday, the president will visit the LBJ Library to commemorate the 60th anniversary of the Civil Rights Act that LBJ signed as president. The historical parallels will be hard to ignore. Though neither man may have remained in office as long as he might have wished, both left a greater mark as a result.

Mark Updegrove is a presidential historian, ABC News contributor and the author of four books, including "The Last Republicans: Inside the Extraordinary Relationship Between George H.W. Bush and George W. Bush" and "Indomitable Will: LBJ in the Presidency." He is also the CEO of the LBJ Foundation. The opinions expressed in this story are not those of ABC News.