Trying to reform the Democratic Party doesn't seem like an effective strategy for leftists. The Republican Party was already a pretty far right-wing party, so pushing it into reactionism was fairly easy. The Democratic Party is no leftist party. Turning it leftist would involve a complete restructuring of the party, basically turning it into a completely new party, and at that point, leftists might as well use their energy to strengthen a currently existing leftist party, like the PSL