Just Look At Kennedy's Record

Gerard Kennedy on October 19th in reaction to the conservatives environment announcements responded by attacking the lack of substance and innovation in their platform. The whole release can be read here.
This is what a leadership candidate should do. A candidate should act as an opposition leader, because, it is pretty obvious, we are in opposition. Gerard has been repeatedly expressing the concerns of the people of Canada against the Harper government. His most recent is blasting the environment plan of the Conservatives.

"The clean air act is actually a dirty piece of legislation," said Kennedy. "It focuses on pollutants that we already have laws to deal with - if they're enforced - and all but ignores the decisive measures Canadians want and expect for dealing with greenhouse gas emissions."
"Once again, Stephen Harper is trying to change the subject because it doesn't fit within his own narrow vision of Canada. He and his Tory cohorts don't believe in climate change - despite the best scientific evidence - so he's coming up with an environmental do-nothing bill that puts off action, maybe forever," Kennedy said. "We should instead implement clean air standards immediately using existing legislation."

Gerard Kennedy has an extensive environment platform, you can view it here and here. Harper's plan for environment leaves Canadians left out of the government's policies. The Conservatives offer no real improvements to help fix the problems that face Canada and the the World.
Gerard Kennedy also notes that the Conservatives environment policy is:

"especially disappointing in its low expectations and in the way it pits one region against another."
"This is cynical politics, and Canadians will know it. Quebecers say consistently that they want action on climate change, and as someone who lived and worked many years in Alberta, I know that Albertans are way ahead of their Tory MPs on this issue...."
"Taking the easy way out - the Harper way - helps nobody. It's time for a revitalized Liberal Party of Canada that will share Canadians' concerns about the environment, and tackle the tough issues."

This isn't the first open barrage on Harper, Kennedy has made. One of the first grevious misjudgements the Conservatives made was their budget, and Kennedy let them know it. He issued a statement, here, in it he says:

With this Budget, the Harper government abandoned our country's commitments to aboriginal Canadians agreed to in Kelowna - cutting planned funding by 80 per cent .
With this budget, the Harper government abandoned our country's fight against global warming - eliminating every single climate change program.
With this budget, the Harper government has abandoned Canada's students - instead of providing up to $6,000 per student for tuition over four years, this budget provides only $80 for text books. Instead of leading Canadians to a shared national sense of purpose, they chose to recycle the failed rhetoric from Mike Harris' Ontario and left Canadians thinking: 'what does this mean for me' instead of 'what does this mean for my country'.

Following that, Kennedy on his website, gerardkennedy.ca, offers an open invitation to write Stephen Harper a letter to express the need for change in Afghanistan, you can read Kennedy's here. But more examples of Kennedy's assault on Stephen Harper and the conservatives on their Afghanistan stance are plentiful. You can read more here, that is where Kennedy poses:

that Canada can help lead the plan to solve the opium trade and development challenges facing the Afghani people, "The only way we can justify staying in Afghanistan is if we can create a mandate for real success."

Newspapers have illustrated Kennedy's forcefulness in attacking the Conservatives failings, you can read such a reference here.
Gerard Kennedy also attacked Harper in his decision to miss the AIDs Conference, you can read his statement here.
And just recently, just before his response to the Conservative Environment policy, Gerard stood up for farmers by demanding Harper keep the single desk for the Canadian Wheat Board. The CWB has been benefiting Canadian farmers for years, the Conservatives want to privatize it through backroom dealings. Gerard,here, states:

"I know from my own experience that the people who best understand how our food system works are the people facing hunger who I met at the food banks and the people who actually produce the food - our farmers.
"They should debate and decide the future of the Canadian Wheat Board, not Stephen Harper's ideological soulmates. And to put forward enterprising ideas that will help our vital agricultural economy, our farmers should have all the information, pro and con, not just what the government thinks."

It is quite clear Gerard Kennedy is leading the opposition against Stephen Harper. The Liberal Party is already coalescing around Kennedy's stance on Afghanistan. And as I've noted in a previous post, Dion promptly addressed the same concerns as Kennedy in regards to the CWB. In all the example I have put forth above, in his very character and in his policy platforms, Gerard has demonstrated to me that he is the best opposition against Stephen Harper.