The Wikipedia article on Truman is worth reading in its entirety. Since it's easy to do so, I'll paste in the part about his unpopularity, which seems to have been solely connected to the conduct of the Korean War and the decision to fire the beloved Douglas McArthur.

On June 25, 1950, the North Korean People's Army under the command of Kim Il-sung invaded South Korea, precipitating the outbreak of the Korean War. Poorly trained and equipped, without tanks or air support, the South Korean Army was rapidly pushed backwards, quickly losing the capital, Seoul.

Stunned, Truman called for a naval blockade of Korea, which went into effect; while the U.S. Navy no longer possessed sufficient surface ships with which to enforce such a measure, no ships tried to challenge it. Truman promptly urged the United Nations to intervene; it did, authorizing armed defense for the first time in its history. (The Soviet Union was not present at the Security Council vote that approved the measure.) However, Truman decided not to consult with Congress, an error that greatly weakened his position later in the conflict.

In the first four weeks of the conflict, the American infantry forces hastily deployed to Korea proved too few and were under-equipped. The Eighth Army in Japan was forced to recondition World War II Sherman tanks from depots and monuments for use in Korea.

Responding to criticism over readiness, Truman fired his Secretary of Defense, Louis A. Johnson, replacing him with retired general George Marshall. Truman (with UN approval) decided on a roll-back policy—that is, conquest of North Korea. UN forces led by General Douglas MacArthur led the counterattack, scoring a stunning surprise victory with an amphibious landing at the Battle of Inchon that nearly trapped the invaders. UN forces then marched north, toward the Yalu River boundary with China, with the goal of reuniting Korea under UN auspices.

China surprised the UN forces with a large-scale invasion in November. The UN forces were forced back to below the 38th parallel, then recovered; by early 1951 the war became a fierce stalemate at about the 38th parallel where it had begun. UN and U.S. casualties were heavy. Truman rejected MacArthur's request to attack Chinese supply bases north of the Yalu, but MacArthur nevertheless promoted his plan to Republican House leader Joseph Martin, who leaked it to the press. Truman was gravely concerned that further escalation of the war might draw the Soviet Union further into the conflict: it was already supplying weapons and providing warplanes (with Korean markings and Soviet fliers). On April 11, 1951, Truman fired MacArthur from all his commands in Korea and Japan.

Relieving MacArthur of his command was among the least politically popular decisions in presidential history. Truman's approval ratings plummeted, and he faced calls for his impeachment from, among others, Senator Robert Taft. The Chicago Tribune called for immediate impeachment proceedings against Truman:

"President Truman must be impeached and convicted. His hasty and vindictive removal of Gen. MacArthur is the culmination of series of acts which have shown that he is unfit, morally and mentally, for his high office. . . . The American nation has never been in greater danger. It is led by a fool who is surrounded by knaves..."

Fierce criticism from virtually all quarters accused Truman of refusing to shoulder the blame for a war gone sour and blaming his generals instead. MacArthur returned to the United States to a hero's welcome, and, after an address before Congress, was even rumored as a candidate for the presidency.

The war remained a frustrating stalemate for two years, with over 30,000 Americans killed, until a peace agreement restored borders and ended the conflict. In the interim, the difficulties in Korea and the popular outcry against Truman's sacking of MacArthur helped to make the president so unpopular that Democrats started turning to other candidates. In the New Hampshire primary on March 11, 1952, Truman lost to Estes Kefauver, who won the preference poll 19,800 to 15,927 and all eight delegates. Truman was forced to cancel his reelection campaign. In February 1952, Truman's approval mark stood at 22 percent according to Gallup polls, the all-time lowest approval mark for an active American president.

"I fired him [MacArthur] because he wouldn't respect the authority of the President. I didn't fire him because he was a dumb son of a bitch, although he was, but that's not against the law for generals. If it was, half to three-quarters of them would be in jail."

Harry S. Truman, quoted in Time magazine

Then:

In 1956, Truman took a trip to Europe with his wife, and was a sensation. In Britain he received an honorary degree in Civic Law from Oxford University, an event that moved him to tears. He met with his friend Winston Churchill for the last time, and on returning to the U.S., he gave his full support to Adlai Stevenson's second bid for the White House, although he had initially favored Democratic Governor W. Averell Harriman of New York for the nomination.

Upon turning 80, Truman was feted in Washington and asked to address the United States Senate, as part of a new rule that allowed former presidents to be granted "privilege of the floor." Truman was so emotionally overcome by the honor and by his reception that he was barely able to deliver his speech.[119] He also campaigned for senatorial candidates. A bad fall in the bathroom of his home in late 1964 severely limited his physical capabilities, and he was unable to maintain his daily presence at his presidential library.

In 1965, President Lyndon B. Johnson signed the Medicare bill at the Truman Library and gave the first two Medicare cards to Truman and his wife Bess to honor his fight for government health care as president.

Let's see what the shrub can do in four short years to turn himself into a beloved ex-prez, standing in Congress with tears in his eyes as the love of the people rains down upon him.