2/3 of Americans support legalizing cannabis. I don’t understand why this bill is so bipartisan among politicians if so many citizens support it.
Because Republicans don't need 50% of the support. They need way less because of imbalances built into the system. If your core support still does not support legalizing cannabis then it's not worth it to do it.
To block an conviction of an impeached president or a constitutional amendment, only 34 senators (17 states) are needed. As of 2018, that was as little as 7.32% of the US population.
If you care to do the math yourself, this is my source: https://simple.wikipedia.org/wiki/List_of_U.S._states_by_pop...
And some python to help once you have lines to summarize: sum(map(float, open('foo').readlines()))