To start with, Michael Den Tandt's theory about the NDP having any interest in propping up continued Con government is utterly out to lunch. But CuriosityCat's Lib spin is far from the right way to look at the NDP's position as well.
No, Jack Layton's tenure as leader (and rise to the position of Leader of the Opposition) isn't a cautionary tale. And that's precisely because Layton refused to make the type of deal Den Tandt sees as possible.
Here's Layton's first-hand account as to what happened when discussions after the 2004 election shifted from merely amending the Throne Speech, and turned to the possibility that Stephen Harper could become prime minister as head of a new government (Speaking Out Louder at p. 341-342):
I asked Mr. Duceppe what he thought would happen if the prime minister refused to accept such an ultimatum. He replied that a government defeat so soon after a general election meant the Governor General would have to turn "to one of us" to form a government. We both knew that meant Stephen Harper and his Conservatives. I asked Mr. Duceppe if he could accept such an eventuality. He was not only clear that he could, but he would.So Layton rightly concluded that installing the Cons in power was antithetical to the values he had been elected to promote. And he held to that position throughout the minority Parliaments from 2004 to 2011 - while the Bloc and Libs took turns supporting Harper (or running for the hills) when faced with opportunities to avoid Con government through a vote in Parliament.
Stephen Harper, while less inclined to brinksmanship, nevertheless warmed to the seduction of Mr. Duceppe's strategy. Under this scenario, Mr. Harper would become prime minister in an informal alliance with the Bloc. Unthinkable? Not to either Mr. Harper or Mr. Duceppe. The Bloc leader was willing to strategize for Stephen Harper to become prime minister, despite the Conservatives' many negative policies...Mr. Duceppe and the Bloc would have been key players in any Harper coalition, demanding significant dismantling of our collective capacities as Canadians as the price of his support. That dismantling was something that would coincide nicely with Mr. Harper's ideological and visceral distaste for any federal government oversight or ability to intervene in any social or economic programs administered by the provinces but utilizing federal tax dollars.
Realizing immediately the full magnitude of what was at stake, I knew I had to walk away. I was not about to participate in any scheme cooked up by the Bloc and the Conservatives that would put the country in the hands of Stephen Harper.
There's no reason to think the NDP would change its view from the position it has held since 2004, as Thomas Mulcair has taken up Layton's mantle in defending the concept of a coalition in pursuit of progressive government. And if anything, the large group of Quebec MPs elected in no small part to maximize the chance of building an alternative government would have all the more reason to hold to the position.
We can thus expect the NDP to be strongly motivated to remove Harper from power if any opportunity presents itself.
And as I've noted before, there should be ample room for a deal between an NDP which is primarily focused on ensuring progressive policy outcomes, and a Lib party which is built primarily around personal advancement (and which is prepared to change its policies at the drop of a hat in pursuit of that end).
If the NDP ranks ahead of the Libs with enough combined seats to form government, it will be in a position to offer Justin Trudeau and his entourage a place in the cabinet to start shedding their "inexperienced" label - and likely wouldn't have much trouble fitting a prominent Lib platform plank or two into a governing agenda.
Similarly, if the Libs finish with more seats than the NDP, there's reason to expect the NDP to focus on having as much of its platform as possible implemented, while the Libs would try to maintain as much personal profile as possible while offering enough of a role in Cabinet to satisfy (and make use of) the NDP's strongest performers.
Of course, it's not clear that Trudeau shares Michael Ignatieff's intention of shedding the Cons' government given the opportunity. And that's where there's some significant risk for progressive voters: the stronger the Libs' perceived likelihood of approaching a majority in a subsequent election, the greater the danger that they'll leave Harper in power.
But there's plenty of reason to think it will be possible for the NDP and the Libs to work out a deal if both want a change in government. And there's no basis at all to worry that the NDP will be the party holding up that process.