Downside?
With a new leader in place and social/religious conservative Harper desperately relying on the radical BQ to prop him up as PM in return for Mulroney-style handouts to Quebec, the Liberals will forget Martin as quickly as they forgot Chretien, and will go on to form a majority.
Upside is that Canadians finally get a majority government again and CPC will have to wait a little longer to get a palatable leader or better still, transform into the PCs.
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Resolved, a minority govt under Harper will be a disaster for the CPC and the county. What if the current polls are right and Martin is returned with another NDP propped govt and, in short order, Tories anoint Peter McKay as leader?
I wonder if your dynamic of fresh new CPC leader vs. tired, can't-seem-to-win Martin, hold true? McKay could be expected to disavow Harper's strict stance on SSM, close ties to USA etc. This might be Tories earliest winning scenario.