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I found by accident an advice that Dean Acheson gave to Harry Truman in 1960 during the primaries (Truman was not enthusiastic about Kennedy). Some Democrats this year would have been well inspired to listen to the old wise man!
" ABOUT OTHER DEMOCRATIC CANDIDATES
" (a) Never say that any of them is not qualified to be President.
" (b) Never say that any of them can't win.
" (c) Never suggest that any of them is the tool of any group or interest, or is not a true blue liberal, or has (or has used) more money than another."
p184-185 of Among Friends. Personal letters of Dean Acheson, 1980