> Either Canada would immediately be the largest state and deliver a massive House majority to one obvious party or, if the provinces joined separately, an overwhelming Senatorial majority to that same party. I don't think that's desirable for the US.

Americans on either side of politics think that Canada is full of super-leftists (and there is no shortage), and Canadians love repeating the "Canadian Conservatives are more leftist than American Democrats" nonsense. But were Canada a part of the US in 2016, Trump would have won AB, SK, and quite possibly enough of the GTA (the parts that loved Rob Ford, and as "Ford Country" has repeatedly won the province for Doug Ford) to win ON, the province most resembling MI/WI/PA, the three states that Trump unexpectedly won the election with. 2024? Very good chance he also takes some/all of BC, MB, NS, and NL.

Going back to southern Ontario, without the NDP-voting north, it is even more a swing state that the GOP has a chance to win.