One of the central debates in the Liberal leadership is how well the various candidates can appeal to the "soft" left. Any criticism of Rae is invariably met with the counter that he has the ability to siphon votes from the NDP. You could also argue that Kennedy and Dion have potential appeal. The big loser in this discussion seems to be Ignatieff, mostly because of the fact that some of his positions lend themselves to the "Harper-lite" accusation.
It is intriguing to look at the possibilities of a Liberal Party with Rae at the helm. Just imagine the problems Rae would present for Layton. Can Layton really attack Rae's record as Premier effectively? The only NDP led government in Ontario's history questioned by an NDPer, that can only dream of such heights. Rae's achilles heel may be rendered unusable through sheer awkwardness. Layton will also lose his most trusted tactic, painting the Liberal Party as convenient progressives, who only play the card during election campaigns. This strategy might have worked against Martin, I doubt it will have much resonance with Rae. Therefore, if you limit your argument to the left of the spectrum, there is little doubt Rae looks problematic for the NDP.
However, it is important to look at the entire political spectrum when accessing a candidate. You can make a powerful argument that Rae's strength on the left, is counter-balanced by his weakness in the middle. While Layton might have problems with Rae, Harper would seem to drool on first blush. I envision a campaign riddled with re-hash and past "experience", particularly in the battleground that will ultimately decide the election. Rae in Ontario is risky, any objective reading surely sees the potential pitfalls. I'm not suggesting Rae can't overcome, but nor do I put much stock in opinion polls from basically disinterested and disengaged people. Maybe Rae can unite the left to a degree, but maybe Ignatieff can unite the middle. An open question which is more crucial?
I now live in Ontario. My experience tells me that many people literally wince, like they are in physical pain, when you mention Rae's name. You can't spin that reality, it's real and it has a firmness to it that troubles me. Anyone reading this blog is a junkie, who knows the debate intimately. The average voter isn't engaged like the diehards, superficial appearances matter and aren't easily changed. While it sounds great to say "unite the left", it has a tinge of idealism that forgets the reality of "lose the middle". It's a double-edged sword.