Here's some free advice for a couple of would-be Canadian prime ministers who are both in the news these days, the NDP's Tom Mulcair and the Liberals' Justin Trudeau: Hammer Prime Minister Stephen Harper on the economy.

Both of them were with their party faithful yesterday -- Mulcair at the final day of the NDP's policy convention in Montreal and Trudeau at his coronation as Liberal leader in the evening in Ottawa. Either of them, it is said here, has the potential to form the next government of Canada if the planets line up the right way.

But that means, to succeed, something is going to have to go wrong for Harper -- because, as we pretty well all know, more often than not opposition parties don't win elections so much as governments lose them.