Hubert Humphrey accepted the second slot to Lyndon Johnson, but it proved more a benefit for Johnson than for Humphrey. Johnson won over liberals, who were otherwise uncertain or displeased about the Southerner who gutted the 1957 Civil Rights Act and felt too close to Southern segregationist Democrats. For his part, Humphrey found himself at the whims of a president who was easy to offend and liked to bully. Humphrey was vindictively frozen out or sidelined for any criticisms of the unraveling Vietnam War, and he found himself as miserable in the Vice Presidency as Johnson perhaps was under Kennedy. He was not permitted to speak in opposition to Johnson's policies, whether within the administration and to the president, or to the press and public. As a result, he gained the reputation of believing the same policies Johnson did in regards to the war, and being a puppet for the president and a 'yes man'. This was devastating as he attempted to seek the 1968 Democratic nomination, and it took his rejecting the administration's then present prosecution of the war to change things.
Upon losing the election of 1968, Humphrey found himself in the wilderness and without a political seat. He won McCarthy's Senate seat in 1971, starting out from the bottom without seniority or committee assignments, without tenure, and without his former contacts. He failed to win the presidential nomination in 1972 and 1976, with his vice presidency serving as an albatross and a point of revulsion against him among many on the left.
On the whole, Humphrey's vice presidency was an opportunity that had potential, but proved to be a set back. It helped Johnson, but it did little for Humphrey, who would have been better served and happier continuing in the Senate. It would also have opened up the possibility of Humphrey running for the presidency on his terms, and not in the light of the Johnson administration and it's war in Southeast Asia, and that entire social context for Americans. Therefore, what if Hubert Humphrey had not been Vice President?