As Andrew notes, "Martin's slide has a noticeable starting point: December 15-16...What happened December 15-16? The debates."
The Globe and Mail editorial board said of that special moment in the debate:
Sean Fine, 9:36 p.m.: That's better. "This is my country." (Paul Martin) now we're talking. Marcus Gee, 9:37 p.m.: Bravo Martin: You're not going to take my country away with some trick, Duceppe. Great, stirring stuff.
Looks like it made an impresson in these elites, but fell flat and made little impression on Quebecers and English Canadians.
In hindsight, the Globe and Mail editorialists pretending to pose as average Canadians just didn't match Canadians' perceptions of the event. This is what they said at the time:
"My impression is that Martin ... had by far the best moments -- particularly his ''you won't take away my Canada" attack on Duceppe. Harper managed not to look scary or sarcastic but failed to break his image as a somewhat detached and over-rational type."
Well at some point he started to look less scary. According to this graph, it was during the debates.
As I said after the first debate regarding this Globe analysis, "Besides the usual pro-Liberal rhetoric of the Globe, what you have here is a bunch of media elites trying to fit into the shoes of a so-called "average Canadian."