- legislation proposed and passed - elections won (i.e. candidate WAR) - ability to make backroom deals - campaigning skills
He's not good at any of those except the last one. I've mentioned the bills, he lost two primaries (his staffers engaged in election denialism both times), only has a seat because he's in a safe Dem state, and has a reputation for being a difficult politician to work with. He is obviously out-of-touch with American voters, his policy proposals are very fluffy and barely substantial, and about the nicest thing I can about his campaigns is that they draw in an enthusiastic crowd, but not one that seems to vote for him and his candidates.
Progressives have much better politicians now: Mamdani, AOC, Al-Sayed. If all of them can deliver like Mamdani - who might genuinely have the best political skills of any American politician living now - then maybe this "what if Bernie won" stuff could actually work. But Sanders would have crashed and burned the second the GOP shut down the government, because he's not good at convincing people who disagree with him.