Camp supporter Roy McMurtry, and accosted Brian Mulroney in a hallway. He resented the leadership of Diefenbaker's successor, Robert Stanfield, describing him as "a very, very sad choice". Horner worked to undermine Stanfield's leadership through manoeuvers such as leading a revolt against the party's support for the Official Languages Act. He was a candidate for the PC Party leadership at the 1976 convention. At one point during the convention, he knocked over an eavesdropping reporter. He finished fourth in the contest, and threw his support to Claude Wagner, who lost on the final ballot to Joe Clark. Horner had even less respect for Clark than he had for Stanfield. Although Clark was a fellow Albertan, Horner regarded him as a city slicker. Horner once paid Clark the ultimate rancher's insult by describing him as a "sheep herder." On April 20, 1977, Horner shocked his constituents and many political observers by crossing the floor to join the Liberal Party, which was at the time deeply unpopular in Alberta. The next day, he joined Pierre Trudeau's Cabinet as minister without portfolio, and was promoted in September 1977 to the position of Minister of Industry, Trade and Commerce. When Diefenbaker, who was still in Parliament 14 years after leaving the Prime Ministership, learned of Horner's defection, he said, "the sheriff has joined the rustlers." Despite (or perhaps due to) his Cabinet position, Horner's floor-crossing was extremely unpopular in his strongly conservative riding. Even by rural Alberta standards, Crowfoot was an unshakably conservative area. Most of its living residents had never been represented by a Liberal, and the bulk of its territory had been represented by centre-right MPs since 1935. He was not helped by the redistribution ahead of the 1979 federal election, which pushed him into a riding that was almost two-thirds new to him. Although it retained the name of Crowfoot, most of its constituents came from neighbouring Battle River, represented by fellow Tory Arnold Malone. Many critics argued that Horner's floor crossing was motivated by opportunism as opposed to ideology and ultimately provoked by the pending electoral redistribution. Had Horner remained with the Tories, the redistribution meant he would have had to face Malone for the PC nomination. Horner would have faced a difficult road to gaining the PC nomination in the redistributed Crowfoot. It was more Malone's riding than Horner's, and Malone was also favoured by Clark and the PC