The NDP, Liberals and Bloq almost did it, only stopped by Harper's proroguing parliament. Wasn't it politically difficult then?
The Liberals won't want to be be relegated to 3rd party status as seems to be the case. The solution is to participate in the next government, something that a coalition will allow them to do.
They'll likely be in power for 4 years which will allow them to rebuild the party. If they decide not to run as a coalition party in the next election, they'll surely find a breaking point which will trigger an election, providing a significant differentiation between the two parties and again making them rivals.