Safire said it first
From The Lord Feldman of Frognal Sir: Much as I enjoyed the positive tone of Howard I. Shapiro's letter on the state of Iraq (25 October), I must tell him that he is wrong in one important detail.
Spiro Agnew did not coin the phrase 'nattering nabobs of negativism'. It was coined by William Safire, my eminent brother-inlaw, in 1970, when he was one of Nixon's speechwriters. The vice-president later credited him with this phrase.