For many years, Liberals have been eager advocates of “strategic voting” – the idea that it makes sense to vote for a party you don't support in order to avoid the election of a party your don't support even more.
A pitch to this effect has been part of the closing argument of every federal Liberal campaign since at least 1993.
http://www.theglobeandmail.com/news/politics/second-reading/brian-topp/liberals-across-canada-are-being-hoist-with-their-own-petard/article2417189/ That might be true. However, there is little evidence that strategic voting played a role in 1997, 2000, and 2008 elections. 1993 and 2011 are a different matter. As the PC vote began to collapse in 1993, many traditional PC voters went in search of a new home and no doubt strategic considerations, certainly region ones, played a role for many. In Western Canada most followed the lead of the NDP voters, who had migrated over to the Reform party prior to the writ being dropped. (The NDP vote collapsed before the PC vote did in 1993. The PCs were tied with Libs going into the 1993 election. That was in marked constrast to the NDP. Going into the 1993 election the NDP were at 8% in the polls. They finished with 7% of the vote. In other words, the notion that NDP voters moved over to the Libs to block the Reform party is not there in 1993. As for 1997 and 2000, the regional makeup of Canadian politics, the unpopularlity of NDP governments in Ontario and BC and NDP support for the Charlottetown accord explain why the NDP vote did not return to normal until 2004. Strategic voting had nothing to do with it.) However, a sizable chunk of PC voters in Western Canada moved over to the Liberals. Most PC voters migrated to the Reform party in Ontario and in the Maritimes the PC vote moved over to the Liberals in NFLD and PEI and to the Reform party to lesser extent in NS and NB. Finally, in Quebec the PC vote moved over the Bloc.
Something similar happened in Ontario in 2011. As the NDP began to surge in Quebec, the Liberal vote in and around Toronto collapsed. Suburban Liberal voters moved in droves to the Conservatives and urban Liberal voters moved in droves to the NDP.
As for the 2004 election, two important things happened. One Ontario voters returned to the NDP after an 11 year hiatus. Between 1965 and 1993 the NDP vote in Ontario never diped below 19% and never topped 22%. The party's share of the vote was very predictable. However in 1993 the NDP took only 6% of the the vote and their share of the vote stayed low for the next two elections. They took 10% in 1997 and 8% in 2000. Then in 2004 they went up to 18%. They took 19.5 in 2006 and 18% in 2008. Something similar happened in BC. Two, the Conservative party vote in 2004 was 6 points lower than the combined PC and Reform vote in each of the three subsequent elections and 7 points lower than than PC vote share in 1988 and 1984 election. In 2006 the conservative vote returned to a historically normal level.
In sum, voters think about voting strategically, but as a rule thumb only when their preferred party's election fortunes collapse mid election. The party's core will go down with the ship, but a sizable number of other supporters, like rats on a sinking ship, will seek refuge with other parties.