I think the NDP has badly miscalculated by drawing Stronach into the ethics investigation. The obvious motivation appears to be an attempt to eliminate the partisan aspect of the story, using Stronach to portray an even standard. If New Democrat MP Pat Martin's desire for a joint investigation is simply a personal view, without political consideration, then he is entitled to his opinion. However, if the NDP has done the political calculation on this issue, and I suspect they have, then they have made a mistake.
First off, the timing is terrible. Why distract from Emerson by introducing the media darling Stronach, in effect clouding the waters and letting Harper off the hook? I don't think the case against Emerson is particularly strong, but Harper has turned this inquiry into a question of accountability and adherence to law. Introducing Stronach changes the channel, which is stupid politically. The first rule of politics is if your opponent is shooting himself in the foot, get out of the way.
The NDP also opens themselves up on the ethical front. Why wasn't this matter engaged at the time of Stronach's crossing? It would seem the importance of this defection has a direct co-relation to the NDP's own political reality. There is no formal alliance with the Grits now so Stronach is fair game, but during the last parliament the NDP was protecting their own interests, first and foremost. Introducing Stronach well beyond any reasonable timeframe makes the NDP look opportunistic and adds a cynicism to the entire inquiry. If the NDP is banking on portraying themselves as the only "pure" party on ethics, the dynamics of the Stronach angle suggest otherwise.
There are certain aspects to the Emerson affair that are unique, with no resemblance to the Stronach matter. What the NDP has done is blend the two so that any discussion of one necessitates the comparison. On substance, the NDP may be right on Stronach, but politically I don't see the benefits to offset the obvious negatives.