If NDP wins more seats than liberals and cons have the most seats, it would be a strategic mistake for the liberals to prop up Harper government. In this scenario NDP would be the official and only opposition to Harper government. It would essentially become NDP vs Con, Left vs Right. Liberals would become irrelevant as a national party. Currently their support is limited to Ontario and I would think they would lose supporters who will see no difference between Liberals/Cons. NDP could easily make the case in Liberal strongholds that essentially voting for Liberals is not much different from Cons.
The best scenario in this bad spot for them is for the NDP to win 100+ seats. In this scenario they can support but not officially join in coalition with the NDP. Let them rule for 4 years or so and than once the NDP shoot themselves in the foot in governing, take the centrist position . Contrast NDP/Con rule and remind the public the good days of liberal government.
