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.