If one morning I walked on top of the water across the Potomac River, the headline that afternoon would read: “President Can’t Swim.”

Lyndon B. Johnson (1908-1973) American politician, educator, US President (1963-69)

(Attributed)

Frequently attributed to Johnson, usually referencing the last year or two of his presidency, when public opinion turned against him over Vietnam. I am unable to find any actual citable source.