Via Matt Fletcher's Living in a Society blog, I read that Stephen Harper has equated the prospect of NDP support in a coalition government to collaboration with the Bloc.
I don't think this works -- either in terms of fairness or as a strategy.
First off, fairness. Well, Matt has this pretty well covered, and POGGe does a good job too. It's not fair to accuse the NDP of being anti-Canada. Or, if disagreeing with one's policy is enough to justify calling them "dangerous to Canada", it would be very easy to accuse the Conservatives of the same thing. After all, with Stephen Harper in power, our armed forces would be under George W. Bush's command in Iraq.
But strategically this is also a bad move, in my opinion, and not merely because discouraging NDP votes likely creates more Liberal votes. If Harper tries to create the impression that the sane choices are those that are closer to the centre, this sort of logic could come back to easily exclude the Conservative Party as well. If there can be extremists on the left, who is going to believe that there can't be extremists on the right, as well?
Now, I suppose it could work the other way. If you want to support the idea that Paul Martin and his party are too left-wing, then by definition the NDP must be wackos. That can fit into a Conservative Party positioning, albeit a very ideological one. Core Alliance types -- see Andrew Coyne's commenters -- might agree and see this as obvious.
On the other hand, I think a different sort of dynamic might work better for the Conservative party. If they think of the NDP as just another party -- one that may even have some advantages over the Liberals, as opposed to merely being a more repugnant and more extreme version of the Liberals -- they may be able to compete against the NDP and even win a couple converts. More importantly, they can try to redraw the map away from one with the Conservatives and NDP as extremes and the Liberals comfortably in the centre.