They aren’t the ones who are undoing the achievements. The rich rightwingers are doing it. They employ a bunch of different channels, including some amount of useful idiots on the left, who are doing their bidding without understanding.
The right can do it because they aren’t bound by any morals, or adherence to decorum, or fear of retaliation, or fear of change in general. Dems are the party of moderate gradual adherence to status quo and maybe moderate change for slightly better, if it’s not too bothersome. And the left for some reason hates the second more that the first, so they aren’t helping.