While I don't read the OP, I'll respond to the thread title, that JFK overrode and greatly offended his labor supporters and his own closest brother and campaign manager, Robert, by picking LBJ.
JFK and LBJ had little use for each other. RFK and LBJ even less. JFK tried to throw LBJ some bones sometimes, like a role in the space program, but he was hardly an advise JFK respected. A private JFK quote summarizes his view: 'Oh, God, can you ever imagine what would happen to the country if Lyndon was president?'
I can't find the video now, but there is a chilling interview of LBJ as VP in which he refers to JFK as "that fellow", as of they are strangers, rather than "the president".
Why JFK chose LBJ: he didn't want or plan to. His choice was Stuart Symington. At the last minute he did, and there are two theories. One is that JFK felt he was the pest person to help him win the general election, partly with Johnson being from Texas to help with the south, and to carry that state. The other is that LBJ blackmailed him.
It is reported that JFK and RFK made an agreement to keep the reasons for the decision secret forever, as if there is some nefarious part of it. RFK reportedly went to LBJ demanding that he refuse the invitation, but JFK refused him.
LBJ felt he was in JFK's shadow, and he very much wanted his own election in 1964 to feel freer. Why did LBJ accept? IMO, it's because LBJ really wanted to be president, and he viewed it as a nearly 10% chance Kennedy's VP would become president. I thought I recall a quote of him saying as much, but can't find it.
