The allusion was, I believe, to a remark reportedly made by LBJ of then-Congressman Gerald Ford, to the effect that the future accidental president could not walk and chew gum at the same time. Actually, LBJ said that Ford couldn't fart and chew gum simultaneously, but this was bowdlerized in deference to the public's then more delicate sensibilities--cf John Nance Garner on the value of the vice-presidency, still widely-quoted as "a pitcher of warm spit" in lieu of the saltier fluid that appears in unexpurgated citations (the Oxford Dictionary of Quotations, for example).

cordially,