He was initially a member of the Ulster Unionist Party but split from them in 1987 when he refused to withdraw from the General Election of that year and instead stood against another Unionist politician. He ran as a "Real Unionist" but failed to win a seat. In the 1995 by-election in North Down he was successfully elected as a "UK Unionist" and he subsequently established his party further in preparation for elections to the Northern Ireland peace talks. He retained his seat in the 1997 election, but lost it in the 2001 election to a UUP candidate.
McCartney is committed to a policy of integration for Northern Ireland, whereby all political and administrative forms of devolution would be wound up, there would be no Northern Ireland wide legislative and administrative assembley and the province would be a full integrated part of the United Kingdom. This proposal, once popular in some sections of Unionism, has increasingly receded and with the recent introduction of devolution to Scotland and Wales, as well as proposals for an English Parliament such a proposal seems increasingly unviable.