Did Lyndon Johnson Underperform in 1964?

[President Johnson]

We have to take into account that blacks weren't allowed to vote in the deep south, so winning there would have been much harder. LBJ did better with the white southern vote than any democrat afterwards, even Carter (just not in the deep south).

LBJ would have done better, had more blacks been able to vote. The African American vote for him was around 95% due to his efforts for civil rights legislation. That was a significant increase since Nixon got around 40% of their votes in 1960.