 Following Labor's election win during 1929, Brennan became Attorney-General in the Scullin Ministry, but was not particularly effective in this role.In any event, he lost his seat in the 1931 election as a result of the swing against Labor, which came about because of the Great Depression and the resultant split in the Labor Party.Although Brennan had gone into the election holding Batman with a comfortably safe majority of 25.8 percent, he was defeated by United Australia Party challenger Samuel Dennis on an unheard-of swing (the biggest in Australian electoral history until that time) of 26.6 percent.