In March 2004, Riddell was approached by the Conservative Party of Canada (CPC) to run as its candidate in the riding of Ottawa South, which was being vacated by Deputy Prime Minister, John Manley. He handily defeated others, including former Ottawa mayoralty candidate Terry Kilrea, for the 2004 CPC nomination. In the ensuing election, he ran against two nationally known names: David McGuinty, the younger brother of Ontario Liberal Party Premier Dalton McGuinty, and Monia Mazigh, of the New Democratic Party (NDP), best known as the wife of wrongfully imprisoned Syrian-Canadian Maher Arar. Although the riding was traditionally Liberal, it was thought that Riddell's background and dynamism, together with voter backlash against Liberal broken promises, gave him a good chance of winning.