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Diamond was an active nationalist before the Second World War and in 1933 was sentenced to a month in jail for refusing to pay a fine given out for addressing an illegal rally in support of republican prisoners. The following year, he stood in the Belfast Central by-election as an "Anti-Partition" candidate.
Diamond was first elected in the 1940s at a time when the Irish Labour Party was in government in the Republic of Ireland. He left the Labour Party because of the (indirect) consequences of that government's travails over the Mother and Child Scheme and the demand of the majority of Irish Labour members in Belfast that the party should challenge the entrenched power of the Catholic Church in Ireland. Diamond opposed these demands and left the party to fight a city council by-election against a Protestant Irish Labour Party supporter of reform. Diamond lost the by-election but viciously attacked the Labour Party for being in the grip of Communism, thereby effectively destroying it as a political force.
In 1944, Diamond became a founder member of the Socialist Republican Party. He took the Belfast Falls seat at the Northern Ireland general election, 1945. In 1949, no-one else contested the seat, and he remained the Socialist Republican Party's only MP, but the party disbanded that year. While most of its members joined the Irish Labour Party, Diamond held his seat, until 1964 as an independent standing as "Republican Labour".
In 1964, Diamond joined with Gerry Fitt to found the Republican Labour Party. He stood for the Westminster constituency of Belfast West (UK Parliament constituency) in the October 1964 general election, but was defeated by James Kilfedder. The campaign was disfigured by riots after Ian Paisley attempted to lead a Unionist crowd up Divis Street to remove an Irish tricolour from the offices of Sinn Féin, who were supporting the candidacy of Billy McMillen. The riots galvanised the Unionist vote while the nationalist vote was split between Diamond and McMillen. Diamond alleged that the official Ulster Unionist Party had colluded in this.
Diamond saw a strong challenge to his position from the resurgent Northern Ireland Labour Party (NILP) in the late 1960s, and was criticised for being inattentive to his constituents' needs. Paddy Devlin of the NILP, a former supporter of the reformists in the Belfast branch of the Irish Labour Party, defeated him in the Northern Ireland general election, 1969.
In 1970, Fitt left the Republican Labour Party to found the broader-based Social Democratic and Labour Party. Paddy Kennedy assumed the leadership, but the party was in terminal decline and disbanded in 1973.
-  Archived December 27, 2005, at the Wayback Machine.
- "Northern Ireland Parliamentary Elections Results: Biographies". Election.demon.co.uk. Retrieved 2013-11-24.
- "Ireland and the Spanish Civil War - Who was Brendan Kielty?". Web.archive.org. Archived from the original on October 28, 2009. Retrieved 2013-11-24.
- "CAIN: Andrew Boyd. (1969) Holy War in Belfast". Cain.ulst.ac.uk. Retrieved 2013-11-24.
|Parliament of Northern Ireland|
|Member of Parliament for Belfast Falls
1945 - 1969