I believe that a Canadian independence party could be a thing, but the problem is it gaining traction. The Canadians lost the 1st Great War, created a headache for the Americans post-GW, and then rose up in rebellion after some prodding by the British and Confederates. I'm assuming that the most recent rebellion gets ground down, or the US decides to threaten the Canadian rebels with nuclear weapons and grants some sort of concession to the Canadians in exchange for surrendering.
So, the Canadians have, within 20 years, fought the US in two major conflicts, losing outright the first time and possibly worse the second time around. Many Canadians might just see the writing on the wall and accept annexation. In other words, I see this party as a mostly fringe group, maybe gaining some support in the latter 20th century, but not enough to be considered relevant.