But if you look at the demographics of who supports legalization, it's clear which party will eventually pass legislation to legalize. Republicans are increasingly dependant on their aging, white base, and that's the exact demographic that still supports prohibition.
The Republican party as a whole has no reason to support legalization until they decide to try to broaden their appeal, which exactly what they planned on doing after their defeat in 2012. But the rise of Trump changed that. Look at who Trump appointed as his first Attorney General--a man who pined for the Reagan era war on drugs--a man who urged prosecutors to use the death penalty to fight drug trafficking.
If they lose in 2020 and decide to rebrand, who knows, but as of right now there is no chance of a Republican Senate passing legalisation, and I think a small but non-zero chance that a Democratic Senate would.