As leader, Singh fumbled early on to take down the Liberal/Conservative smears that he was a a "terrorist sympathizer." But the party was struggling longbefore he won the leadership. The NDP woes began before the last election, which was a complete failure. It m ay have been the second best result for the party, but this ignores the fact that the party was literally in majority territory for two weeks in July and then saw a decline of up to 30% in support in provinces like Quebec in the lead up to the election vs election day. Mulcair was a terrible leader who was handed the perfect storm to present at the very least a social democratic alternative to both the Liberals and Conservatives who were tanking in support mainly because of their support for austerity and C-51 (anti-terrorism legislation). He squandered that opportunity. He discredited himself and the party by tying himself to a insipid platform while simultaneously claiming to be the next Tommy Douglas. Tommy Douglas' vision was a dumb platform with centerpieces like an innovation tax credit and balanced budgets? No. However that's not to legitimize the Liberal myth that their platform was more progressive, it was not.

Outside of Pat Strogan, I do not think any of the leaders would have been able to rehabilitate the NDP in one election cycle. When the NDP dies, it takes decades to rebuild. (That's a paraphrase of Brian Topp.)