No earlier than ww1? Hard, but I'll try.
1. During 1914, lots of the French-speaking Walloons escaping from the German offensive (more than OTL) are sent to Quebec instead of France. Population increases.
2. USA is isolationist, not only towards European politics, but also closes itself to immigration. The flow of immigrants that came in the post-ww1 turmoil to the peaceful Americas end up in Canada instead.
3. The depression hits the USA a lot harder than OTL, while Canada is off lighter. With a more vibrant economy and a larger population than OTL, Canada attracts many unemployed citizens of the USA.
4. More corruption and a stronger organised crime in the US eats up the little post-depression growth there were in OTL.
5. Hoover beats FDR in the elections and continue an isolationist, immigration-unfriendly and economically restrictive policy, keeping the US economy deep in depression. In OTL, there was a lot of socialist and liberals in the US - with a restrictive economical policy and an anti-communist/anit-liberal attitude of the establishment (heighten those pre-ww2 sentiments in the US), many people might feel more at home in a more liberal Canada than a more conservative USA.
6. When ww2 rolls around, Canada is bigger both in population and economy than in OTL. USA is even more isolationist and does not pursue an aggresive foreign policy towards an Imperialistic Japan. I migth even see the US selling Pacific Islands to Canda ("we do not need these non-american territories") in the late 30s.
7. US does not enter ww2 and does not pass any lend-lease or build up industry through war economy. Us citizens who wants to fight for the allies have to enlist in the Canadian armed forces (this happened on a small scale in OTL, imagine it a bit bigger and returning troops to be unwelcome in the US since they have been allied to commies and liberals, being treated as traitors).
8. The commonwealth does well in the war, better than OTL. Canada gets the Polish refugees (Britain or the US does not want them).
9. The US economy has the potential, but never grew to be the powerhouse it was in our TL. Latinos use the US as a transit land to egt to Canda rather than their big dream. The US frowns upon immigration even more post-war.
10. With a ww2 slightly longer (without US participation), the Soviet Union is ravaged even more and hardly powerful enough to occupy eastern Europe, let alone compete for superpower status. The Soviet Unions goes into a decline by the 1960s and fall by the early 1980s as Solidarity paves the way in Poland.
11. Canada is by now bigger in population than the US, if only slightly, and has a more vibrant economy. Since Britain lost so much in ww2 (even if not as much as in OTL), Canada becomes the leading world power, aided by Britain and the rest of the Commonwealth.
