McCombs was part of the committee which drafted the founding constitution and programme of the New Zealand Labour Party in 1916. He then became inaugural president of the Labour Party. The following year, he resigned the presidency and his membership of the Labour Party over the state control of liquor issue. After rejoining the party in 1918, McCombs served as Labour's deputy leader from 1919 until 1923. When Labour's caucus leader Alfred Hindmarsh died during the Influenza epidemic, Labour's leadership was open. McCombs made claim to the title but was opposed by the more militant Harry Holland. The caucus held an election to decide between the two. The result was a tie. After drawing lots, Holland was successful. During the 1920s McCombs with Dan Sullivan led the opposition to Harry Holland within the Parliamentary Labour Party caucus attempting several leadership challenges, all of which were unsuccessful.