Mayhew was a Labour MP for nearly 30 years, serving as a minister under Atlee and Wilson but in 1974 he crossed the floor briefly becoming the 15th Liberal MP. He said that he left Labour because it had become too vulnerable to the extreme left and too dependent on the trade unions. He fought the marginal seat of Bath in the October 1974 election losing by 2,122 votes. Together with those of Dick Taverne, Mayhew’s actions provided a warning of what was to happen to the Labour Party in 1981 with the formation of the SDP. Mayhew became a life peer in 1981.