After the 1974 election, McMahon returned to the backbench for the first time since 1951. In the lead-up to the dismissal of the Whitlam Government in 1975, he strongly defended the power of the Senate to block supply. However, he believed that Governor-General John Kerr had acted unconstitutionally in dismissing the prime minister, and said that he would have challenged the decision in the High Court if he had been in Whitlam's position. McMahon believed that those responsible for the "loans affair" – including Whitlam and several of his ministers – had acted illegally and should be prosecuted for their involvement. He assisted Danny Sankey (a private citizen) in bringing a private prosecution against Whitlam, which eventually came before the High Court as "Sankey v Whitlam". Malcolm Fraser had promised Kerr that his government would bring no action against its predecessor, and was frustrated by McMahon's actions. In his memoirs, he said: "I knew McMahon was running around up to his tricks ... I couldn't control what he did, but I could make damn sure that the government, my government, did not get involved".