Notes from Wikiquote
According to Snopes (David Emery, July 26, 2016), "not all of Ronald MacMillan's anecdotes, when checked, check out" and that furthermore "the historical evidence suggests that far from being concerned about securing future generations of black votes, one of Johnson's main worries—which, to his credit, didn't prevent him from pushing for passage of the Civil Rights Act of 1964—was losing the votes of white Southerners". Snopes concluded that while this quote is not impossible, "on balance we have to question its authenticity".