Has the Democratic Party been moving to the left over the last ten years? Please explain your answer.

I recently had a [disagreement](https://old.reddit.com/r/AskALiberal/comments/1toatmp/for_my_fellow_liberals_are_you_ever_frustrated_by/oo0jj5q/) in this subreddit with u/___AirBuddDwyer___ about this question, and I thought I'd ask the broader subreddit.

I'll give my answer below, but I really want to know the consensus from the rest of you.

(Remember, the question is not 'Is this good or bad?'; the question is more 'Did it happen?'.)