The Vietnam War was what did Lyndon Johnson in. He referred to it as "that bitch". Johnson had made great plans for a renaissance in social welfare legislation and had accomplished much as part of his vision of a "Great Society". But when it came time for Johnson to seek re-election in 1968, the War in Vietnam would not allow it.

Johnson had served less than 24 months of President John F. Kennedy's term, and therefore he was constitutionally allowed to run for a second full term in the 1968 presidential election in accordance with the provisions of the 22nd Amendment to the Constitution. At first, no prominent Democratic candidate was prepared to run against a sitting president of the Democratic Party. Maverick Senator Eugene McCarthy of Minnesota challenged Johnson as an anti-war candidate in the New Hampshire primary, hoping to pressure the Democrats to oppose the Vietnam War. When, on March 12, McCarthy won 42 percent of the primary vote to Johnson's 49 percent, Johnson saw the writing on the wall. This was an amazingly strong showing for a challenger who had been a political nobody. When other political sharks saw Johnson's blood in the water, they joined in the race. On March 16, Senator Robert F. Kennedy of New York entered the race. Internal polling by Johnson's campaign in Wisconsin, the next state to hold a primary, showed that Johnson was trailing badly. Johnson did not leave the White House to campaign because he felt that his duty was to stay put and mind the war.
By this time the Democratic Party was splitting into four factions, each of which generally disliked the other three. The first consisted of Johnson (and Humphrey), labor unions, and local party bosses (led by Chicago Mayor Richard J. Daley). The second group was made up of students and intellectuals who were strongly against the war and who parked their support behind McCarthy. The third group were Catholics, Hispanics and African Americans, who supported Robert Kennedy. The fourth group were white segregationist Southerners, who supported Alabama Governor George C. Wallace and the American Independent Party. Vietnam was one of many issues that splintered the party, and by this time Johnson realized that it was an unwinnable war. He could not conceive of any way to unite the party so as to win re-election.
In a televised speech he gave on March 31, 1968, Johnson surprised many people when he announced he would not run for re-election by concluding with the line: "I shall not seek, and I will not accept, the nomination of my party for another term as your President." The next day, his approval ratings increased from 36% to 49%.
In June of 1968, Robert Kennedy was assassinated after winning the California primary. Johnson rallied the party bosses and unions to give Humphrey the nomination at the 1968 Democratic National Convention. In what was considered by some as an "October surprise", Johnson announced to the nation on October 31, 1968, that he had ordered a complete cessation of "all air, naval and artillery bombardment of North Vietnam", effective November 1, should the Hanoi Government be willing to negotiate and citing progress with the Paris peace talks. Although it gave the campaign of Hubert Humphrey a boost, it was not enough to prevent Richard Nixon from winning the election.
Johnson attended the inauguration of his successor Richard Nixon. According to historian Michael Beschloss, "On Inauguration Day, Johnson saw Nixon sworn in, then got on the plane to fly back to Texas. When the front door of the plane closed, Johnson pulled out a cigarette—his first cigarette he had smoked since his heart attack in 1955. One of his daughters pulled it out of his mouth and said, 'Daddy, what are you doing? You're going to kill yourself.' He took it back and said, 'I've now raised you girls. I've now been President. Now it's my time!' From that point on, he went into a very self-destructive spiral." Other historians claim that he had surreptitiously smoked before that, but whatever the case, it was clear that by this time, Johnson appeared unconcerned about his health. After leaving the presidency in January 1969, Johnson went home to his ranch in Stonewall, Texas. His former aide and speechwriter Harry J. Middleton came along to write Johnson's first book, The Choices We Face, and work with him on his memoirs entitled The Vantage Point: Perspectives of the Presidency 1963–1969. The memoirs wer published in 1971.
Also in 1971, the Lyndon Baines Johnson Library and Museum opened on the campus of The University of Texas at Austin. Johnson donated his Texas ranch in his will to the public to form the Lyndon B. Johnson National Historical Park. He decreed in the will that that the ranch was to "remain a working ranch and not become a sterile relic of the past".
Johnson had once been critical of Nixon during the 1968 election campaign because he believed that Nixon was undermining the Paris Peace Talks during the presidential election campaign. But he was complimentary of Nixon for his foreign policy, though he worried that his successor was being pressured into removing U.S. forces from South Vietnam too quickly and before the South Vietnamese were really able to defend themselves. He said, "If the South falls to the Communists, we can have a serious backlash here at home."
During the 1972 presidential election, Johnson endorsed Democratic presidential nominee George S. McGovern, a senator from South Dakota, even though McGovern had long opposed Johnson's foreign and defense policies. Privately he was unhappy with the McGovern nomination and with his platform and policies. He warned friends that Nixon could be defeated "if only the Democrats don't go too far left." Johnson had thought that Edmund Muskie would have been the best candidate to run against Nixon. Johnson's was disappointed that his friend and fellow Texan John Connally had served as President Nixon's Secretary of the Treasury and then stepped down to head a group called "Democrats for Nixon".
In March 1970, Johnson suffered an angina attack. He was taken to Brooke Army General Hospital on Fort Sam Houston in San Antonio. His doctors advised him to lose some weight. Johnson's weight had increased since leaving the White House. He had grown his hair long and gained over 25 pounds and now weight around 235 pounds. He had also resumed smoking after nearly an absence from the habit of nearly 15 years. Later that summer, Johnson once again began to experience chest pains. He went on a crash water diet, and lost about 15 pounds in less than a month.
Two years later, in April of 1972, Johnson suffered another heart attack while visiting his daughter, Lynda, in Charlottesville, Virginia. When he was visited by friends in hospital, he said, "I'm hurting real bad." His chest pains caused a considerable shortness of breath and he had a portable oxygen tank next to his bed that he would take breaths from while lying down. But despite these problems, he continued to smoke heavily. He was placed on a low-calorie, low-cholesterol diet, but he was not very faitful in sticking to it. He also began experiencing severe abdominal pains. Doctors diagnosed this problem as diverticulosis. X-rats showed pouches of tissue forming on his intestine walls. His condition rapidly worsened and surgery was recommended. Johnson flew to Houston to consult with heart specialist Dr. Michael DeBakey. DeBakey discovered that two of the Johnson's coronary arteries were critically damaged. He concluded that the overall condition of his heart was so poor that attempting a bypass surgery would likely result in fatal complications.
Johnson's heart condition was now diagnosed as terminal, and he returned home to his ranch. At approximately 3:39 pm Central Time on January 22, 1973, Johnson placed a call to the ranch's Secret Service compound complaining of "massive chest pains". The agents rushed to the former President's bedroom and found him unresponsive with the phone receiver still in his hand.The former president was airlifted in one of his own airplanes to San Antonio and taken to Brooke Army General. It was there that Lyndon Johnson was pronounced dead on arrival at the facility by cardiologist and Army colonel Dr. George McGranahan. Johnson was 64 years old when he died.
Very soon after Johnson's death, his press secretary Tom Johnson (no relation) telephoned Walter Cronkite at CBS. At the time Cronkite was live on the air with the CBS Evening News at the time, and a report on Vietnam was cut abruptly while Cronkite was still on the line, so he could break the news. Johnson died just two days after Richard Nixon's second inauguration. Nixon had won a landslide victory in the 1972 election. When Johnson died, it was first time since 1933 that there were no former Presidents still living.

Johnson was given a state funeral held at the National City Christian Church in Washington, D.C., presided over by President Richard Nixon. Johnson was buried in his family cemetery. Although it is part of the Lyndon B. Johnson National Historical Park in Stonewall, Texas, the land is still privately owned by the Johnson family, who have requested that the public not enter the cemetery. He is buried just a few yards from the house in which he was born.