Mid Ulster was represented from the 1951 general election by Michael O'Neill, who ran as an 'Anti-Partition' candidate associated with the Irish Anti-Partition League. However at a Nationalist convention on 8 May 1955, O'Neill's supporters were outnumbered by supporters of Sinn Féin who had picked Tom Mitchell, forcing O'Neill to stand down in his favour. Mitchell was in HM Prison Belfast serving a sentence of ten years' imprisonment after he was caught during an Irish Republican Army raid on Omagh barracks. Although Mitchell as a Sinn Féin candidate did not have access to Nationalist halls to hold meetings, nor to those run by the Roman Catholic Church, he was still regarded as the favourite.