After awhile, when accusations are continually and sweepingly made against all men, good and bad, the public as a whole grow to believe that there is a little something bad about the decent man and that there is not much bad about the crook. No greater harm can be done to the body politic than by those men who, through reckless and indiscriminate accusation of good men and bad men, honest men and dishonest men alike, finally so hopelessly puzzle the public that they do not believe that any man in public life is entirely straight; while, on the other hand, they lose all indignation against the man who really is crooked.

Theodore Roosevelt (1858-1919) American politician, statesman, conservationist, writer, US President (1901-1909)
Speech (1910-08-29) “The Nation and the States,” Colorado State Legislature, Denver
    (Source)

Collected in Roosevelt, The New Nationalism, Part 1 (1910).