Did Lyndon Johnson Underperform in 1964?

[Alben Barkley]

No. He set a record for the popular vote since the popular vote became near-universally adopted. He only lost Deep South apartheid states which prevented his base from voting and (barely) Goldwater's home state where he didn't bother to campaign.

Maybe there's an argument that he could have won AZ/GA/SC/LA (in that order of likelihood) without voter suppression and/or with an even more brutal and vigorous campaign, but I think the record he set, which is unlikely to be surpassed any time soon, was more than enough of a thrashing. Goldwater was always going to get somewhere between 35-40% of the vote simply by virtue of being the Republican nominee and the conservative standard-bearer. Johnson vs. Rockefeller could have been a much more interesting election; Johnson still would have won, but the map would have likely looked quite different (especially in New York!)