You may not think I am an expert on Canada's constitution, but I have been studying the Citizenship Guide for nearly two weeks, so I am on top of this [Um, no, I am not].*
* My stance on the Senate: I am pro-reform, not abolition. Why? Because the Senators should be elected, so that they are accountable Why not abolish? Because it would be nice to have some folks in the Parliament Buildings whose agendas are not completely controlled by party leaders and especially the PM. Of course, elected Senators might have some party loyalty .... Hmmm, anyhow, that is my stance as un-finished as it might be.
Senate-bashing is popular these days due to a variety of scandals, including one that ultimately implicated Harper's closest advisers. And this might be seen as a play for NDP support given that this is an NDP stance and to put the Liberals in an awkward spot of defending the status quo.
I have not read heaps of stuff on this, but I have a question: Harper has left a bunch of Senate seats unfilled, so if the NDP win, they could nominate a whole bunch of Senators at once, going from zero (right?) to a healthy percentage. This is not quite like an American President ending his term without filling vacancies on the Supreme Court, but it is not that different either. Obviously, the big difference is that the Surpreme Court in the US is far more influential than the Canadian Senate. So, perhaps the risk is minor, but I would almost vote for the NDP (if my citizenship comes through in time--it will be close!) just to get to this outcome. Sure, Thomas Mulcair as the new Prime Minister might have to look a bit hypocritical in filling a Senate that he bashed, but he could honestly say that he would be meeting his constitutional obligations.
For a far smarter take on this stuff, see this interview with Emmet Macfarlane.