Bernie Sanders always drew big rallies. Leftists hate Democrats. AOC and Sanders are talking about Medicare for All for the 10th year in a row. Whoever wins the nomination, won't be campaigning on Medicare for All, they will get tepid endorsements from Sanders and AOC at best, and leftists will sit out because they aren't pure enough. We have seen this before.
I'm not sure how holding rallies is fighting. Nothing is changing. Dems across the country are having town halls. AOC has been in congress for 6 years, passed 0 bills, voted against things like capping insulin, and sided with Marjorie Taylor Greene over Russia. She's not the fighter we need.