"Democrats" is an amazingly broad term that mostly encompasses "almost anything that isn't Republicans". It isn't so much a unified monolith as an alliance of often polar opposite interests banding together only because the GOP is the only real alternative. Were the GOP to disappear tomorrow, I guarantee the Democratic party would almost instantly split into dozens of smaller parties, or possibly into two again. Compared to most of Europe, an American "Democrat" is a centrist or even a right wing neoliberal. People like Bernie Sanders, who is viewed as a far-left outright communist in America, would barely count as center left in most of Western Europe.