 Following pressure from the far-left in his local party, O'Halloran was among the Labour MPs who defected to the newly founded Social Democratic Party (SDP) in 1981.However, in 1983, the SDP chose John Grant, the MP for Islington Central (which was being abolished in boundary changes), to be their official candidate in Islington North.O'Halloran left the SDP to stand as an Independent Labour candidate at the 1983 general election.