It is one thing to seize terrain and another to hold it. The American public would be confronted with video images of their sons and daughters being picked off one by one at night by Canadian soldiers, reservists, and citizens who volunteered as paramilitary partisans.
100%. Beating Simo Häyhä's record is a tough challenge, but it gives us a nice goal to aim for.
Plus, it's not like it would be hard to take the fight to them. I imagine appetite for an invasion would fade when American infrastructure became a target.