Dallas happens on October 22, 1964

[Sir Mohamed]

What if Dallas happend on October 22, 1964, less than two weeks before the election? The GOP ticket remains unchanged in that Scenario.

LBJ, of course, becomes president. Does the DNC nominate him immediately after he takes office? If he had been replaced as VP, what happens with the guy?

Does he win an even bigger landslide?

[MASHED POTATOES. VOTE!]

What if Dallas happend on October 22, 1964, less than two weeks before the election? The GOP ticket remains unchanged in that Scenario.

That's a big assumption. However, with Goldwater as the nominee, Kennedy would be probably leading by more or less comfortable margin, barring some devastating scandal. America was simply not ready for a Goldwater-style President in the 60s. Even if Goldwater leads somehow, JFK's assassination changed everything. There's no way LBJ would lose after that.

However, there's another interesting question: JFK seriously toyed with dumbing Johnson in 1964 and mentioned Terry Sanford as possible replacement. So let's assume LBJ did become President via succession in 1964, but had been already dropped from the ticket in favor of someone else. Who would become a default nominee then? Lame-duck VP-turned President or JFK's new running-mate?

[President Johnson]

I doubt that LBJ would have been replaced as Vice President. On October 31, JFK was asked: "Do you intend to keep Johnson and the ticket and do you expect him to be nominated?"; and the president replied "yes to both of these questions".