Question

Why did some Democrats oppose Hubert Humphrey I’m his 1968 race for presidency

Answer 1

In the presidential campaign of 1968 there was a  battle for control of the Democratic Party. On one hand Humphrey with his old politics and, on the other, Mc Carthy and Kennedy with their new politics. The main reason of the division was the Vietnam War. It intensified Humphrey's unpopularity.

Answer 2

Johnson announced that he would not run for a reelection, and that's when Humphrey decided to run, launching his campaign for presidency right after. Considering that he was loyal to Johnson's administration policies considering the Vietnam War, his own party disliked the idea of him running and they opposed to  it, since they disagreed on how Johnson was dealing with it all.