Thatcher appointed Hamilton a whip in July 1990. In November 1990, Michael Heseltine initiated a leadership challenge to Margaret Thatcher. Hamilton was told by the Chief Whip to stay neutral, but says he ignored this instruction. "I naturally ignored this advice and fed all my intelligence into her campaign." He also said that he made the fateful suggestion that she interview each cabinet member individually, believing they would lack the resolve to tell her to her face that she must go. "Unfortunately, I had miscalculated." Hamilton strongly encouraged Thatcher to persist. At a meeting where Peter Lilley argued that Thatcher could not survive, Hamilton subjected him to a barrage of "sarcasm and heckling". On 21 November 1990, Hamilton and like-minded colleagues met Thatcher at Downing Street. Thatcher did subsequently resign, and in the next round of the election Hamilton voted for John Major.