StewartBrian,

The Federal Liberal Party for decades had a significant progressive element that would fit in the NDP (many of whom seriously considered moving over during the Liberal Party's electoral collapses during 2011-15).  In turn, the Liberals have won the votes of many traditional NDP supporters whenever it looked like the Liberals were in a close election with a reactionary Conservative Party. As much as 20% of the electorate constitutes this swing vote.

The Liberal Party adapted to this fluid situation by campaigning well to the left (but maintaining the support o its conservative element by governing more to the centre or even moderate right in Office).

A good measure of Liberal Party periodic concern about possible NDP inroads is the changing volume of their progressive rhetoric.