His declining popularity is finally being compared to a newly nominated Conservative from last month and a NDP non-entity as they're still in the process of selecting their leader. 
Of course he has been the highest in Canada averaging 60% (now around 50%) because he hasn't been compared to any established party leader for any length of time.
That being said his numbers have been declining  +10% over the last year being compared to no other party leader which doesn't bode well for him.
There is a marked difference when you compare the preferred party allegiance which the Conservatives and the NDP are steadily chipping away at the Liberal's 40%