The average population also probably wouldn't go along with it.
Well that's why you don't just invade outright. You need a cassus belli first. Canada unfairly treating the US on trade is a start, but it would need to be developed a bit more. Perhaps a year or so of hammering the negative effects of Canada's unfair trade practice on the US until you can logically make the leap that there is no choice but to invade Canada in order to allow the US to survive.
The message has to be delivered properly for the population to go along with it. Perhaps stage some "grassroots" manifestations against Canada in states where you have senators who are hesitating about supporting the war to pressure them.
Oh I meant the average population in Canada ---
Not removing this until Mega Man 64 is released on the Wii Virtual Console. Started on: 12/1/2009 https://imgur.com/mPvcy