In May 2005, when it appeared that the Liberal government was about to be defeated and another election called, Riddell was recruited to run in Ottawa South a second time. In reaction to rumours that he had once worn a World War II German officer's costume to a campus costume party, as a teenage Halloween prank 28 years earlier, Conservative officials initially blocked Riddell's renomination as the Party's candidate. Riddell's disqualification caused a public outcry from Riddell's supporters across Ottawa, and was quickly assessed by the Conservative Party's National Arbitration Committee.