Question

Why did president Johnson decide not to run for reelection in 1968?

Answer 1

The real reason for Mr. Johnson's withdrawal was Vietnam. It was an involvement he had questioned as a Senator and about which he brooded as President. There were too many casualties. In addition, Mr. Johnson had begun to doubt our ability to prosecute the war to any clear-cut victory.

Answer 2

Public opposition to the Vietnam War had made him unpopular.

President Johnson was a faithful adherent of the Vietnam War but as the opportunity for the polls in 1968 appeared the citizens started investigating the US engagement in the Vietnam War and was largely objected to it. President Johnson apprehended his beliefs turned him shunned and he understood he couldn't muster adequate support and thus did not contested for election again.