Dion ran on an environmental platform in the 2008 federal election, but was defeated by the Conservatives led by Prime Minister Stephen Harper in one of the Liberals' worst electoral showings.

[3] Dion was involved with the sovereignty movement, first as a teenager attending a Jesuit college in Quebec City,[4] and later as a university student campaigning for Parti Québécois candidate Louise Beaudoin in the 1976 election.

)[6] He studied political science at Université Laval in the department co-founded by his father;[7] this was also where he met his future wife, Janine Krieber, a fellow student in the same program.

[citation needed] After Dion obtained BA and MA degrees in 1977 and 1979 respectively (his master's thesis presented an analysis of the evolution of Parti Québécois electoral strategies[9]), he and Krieber departed together for France.

Dion spent four years in Paris, living with Janine in the Montmartre district and studying public administration under the supervision of noted sociologist Michel Crozier.

[10] Dion taught there from 1984 to January 1996, specializing in public administration and organizational analysis and theory, and was a guest scholar at the Brookings Institution in Washington, D.C., during a 1990–91 sabbatical leave.

His decisive conversion to federalism, as he later recounted to journalist Michel Vastel, occurred as he was preparing for a presentation in Washington: I sat down at my computer at 11 o'clock, and, at noon, I had a text that was so interesting that the Americans wanted to publish it.

In his responsibilities as Intergovernmental Affairs minister in the Jean Chrétien government, Dion was tasked with challenging the arguments of the Quebec sovereignty movement much more vigorously than in the pre-referendum period.

The strongest complaints were on the presumed ambiguity of the 1995 question and that Quebec had tabled a law reserving the right for the National Assembly to declare independence unilaterally if constitutional negotiations with the Government of Canada failed.

Against the first assertion, Dion argued that the vast majority of international law experts "believe that the right to declare secession unilaterally does not belong to constituent entities of a democratic country such as Canada.

Quebec sovereigntist leaders were granted a prominent role in the conference and used their floor time to denounce Canadian federalism to an international audience to the great annoyance of their federalist host.

"[27][28] After Paul Martin's assumption of the office of Prime Minister, Dion was dropped from Cabinet as part of a general effort to dissociate the new Liberal government from the outgoing Chrétien administration.

[30] Shortly after his appointment, a Globe and Mail article described Dion as being "bent on transforming the environment dossier from the traditional tree-hugger's last stand into a forward-thinking economic portfolio.".

[31] Dion championed a "new industrial revolution" focused on "environmentally sustainable technologies and products",[32] and he sought to nurture a collaborative relationship with big business rather than a confrontational one.

[39] In April 2005, Dion unveiled his "Project Green" to combat climate change, but the program was immediately criticized by some environmental groups for being too timid and for lacking in meaningful regulations.

[40] Johanne Gélinas, Canada's environment commissioner, criticized the government's stewardship of marine areas and national parks, as well as its efforts to ensure the safety of drinking water.

Dion's level of support was similar to that of former Ontario cabinet minister Gerard Kennedy, both candidates being in a distant third or fourth place, though still significantly higher than the other four leadership contestants.

[52] As rumours circulated of a possible election in early 2007, Dion bolstered the image of a Liberal Party renewed and healed of its internal divisions by appointing many of his former leadership rivals to key campaign positions.

[53] In early January 2007, Dion made a leadership decision in regards to Wajid Khan, a Liberal MP who was serving as a Middle-East adviser to the Prime Minister.

[56] Shortly after Dion led the Liberal caucus in its rejection of the 2007 Conservative budget, arguing that it failed Canadians on economic prosperity, social justice and environmental sustainability.

[59] On 1 February, Dion tabled a motion challenging the Conservatives to reaffirm Canada's commitment to the Kyoto Protocol, attempting to capitalize on a 2002 letter in which Prime Minister Harper described the Accord as a "socialist scheme" that is based on "tentative and contradictory scientific evidence" and designed to suck money out of rich countries.

[62] On 27 February, Dion's Liberals, together with Bloc Québécois and NDP members of Parliament, voted down a Harper government proposal to extend two controversial provisions of the Anti-Terrorism Act for another three years.

Though Ignatieff phoned up Dion to deny the allegations, The Globe and Mail suggested that the report had a negative impact on the Liberals' morale, citing the NDP's widening lead after the article's release.

[84] While the Party was divided on whether or not the government should be toppled on a confidence vote regarding the Throne Speech, Dion mentioned in a statement in the House of Commons on 17 October that the Liberals will support the Throne Speech but with major amendments including the Kyoto Protocol and the end of mission in Afghanistan by 2009 and had criticized the government on several aspects including the economy, seniors and child poverty, the crime policy, the Senate reform.

The plan was immediately criticized by Prime Minister Harper, who described it as a tax grab and compared it to the National Energy Program that the federal Liberal government adopted in the 1980s.

[93][94] On 11 September 2008, NDP leader Jack Layton also criticized the Green Shift, saying that it would hurt consumers, would be nothing more than a nuisance for energy producers, and evaluates emission equally across all sectors instead of maximizing reductions where the cost is lowest.

[107][108] On 4 December 2008, the Governor General granted Prime Minister Harper's request to suspend parliament until January 2009, thereby delaying a scheduled non-confidence vote and the likely defeat of the Conservative government.

[115] In September 2009, it was reported that Denis Coderre, Ignatieff's Quebec lieutenant, was attempting to oust Dion and fellow Montreal-area MPs Raymonde Folco, Lise Zarac, and Bernard Patry for the next election, with the hope of replacing them with star candidates.

[121] The 2015 election was a resounding victory for Justin Trudeau's Liberal Party, and Dion was comfortably re-elected to an eighth term representing the newly reformulated Saint-Laurent riding.

[126] In a March 2016 speech at the University of Ottawa, Dion used "responsible conviction" – a term syncretized from the work of German sociologist Max Weber – to describe the Trudeau government's foreign policy.