leftism is basically socialism and communism, which the democrats are kinda neither
Yeah, they support restrictions on what companies can do and consumer/worker rights, but they still want to keep the capitalist system intact.
Bernie Sanders is sorta on the more socialist side, his campaigns and beliefs are way more anti-corporation. However there were leaked emails from 2016 showing the DNC (basically the core of the democrat party) basically shooting his campaign dead from within so Hillary Clinton would become the leading candidate for the party.
So they aren't really "leftist" just "not on the right"