He was named the PQ's critic in research and development by leader André Boisclair.
In 2005, he backed André Boisclair in the PQ leadership race and was appointed PQ's director general in 2006.
André Previn | André Breton | Andre Agassi | André Malraux | André Derain | Victor André Cornil | Carl Andre | Andre Ward | Fabrizio De André | André the Giant | André Heller | André Gide | André-Gaston Prételat | André | Andre Norton | André Masséna | André Guignard | André Franquin | Andre Begemann | Peter Andre | Andre Williams | Andre Ware | André Leon Talley | André Le Nôtre | André Glucksmann | André Charlot | André Sogliuzzo | André Marie Constant Duméril | André-Jacques Garnerin | André Bauer |
He endorsed Richard Legendre in the 2005 PQ leadership election, which was won by André Boisclair.
In the 2007 election at age 22, Diamond became the youngest member ever elected to the Quebec legislature, a record he held until the 2012 election of Léo Bureau-Blouin; the previous recordholders had been André Boisclair and Claude Charron.
He had faced controversy during the campaign, when radio host Louis Champagne attacked both Gaudreault and Parti Québécois leader André Boisclair for being openly gay.