You would be hard-pressed to find anyone willing to dispute the narrative that Democrats are steadily lurching further Left.
A recent FiveThirtyEight review of relevant data contended that "[t]oday’s Democrats have multiple identities but increasingly one position on identity issues: the liberal one."
I'm not sure the entire piece draws accurate conclusions, but its focus on one particular shift among Democrats seemed especially telling — the Dream Act. From FiveThirtyEight's story:
In 2010, five Democratic senators — all of whom were white — voted against the Dream Act, which would have created a path to citizenship for undocumented immigrants brought to the U.S. as children, whose status is now one of the the reasons for the congressional shutdown impasse. Eight years later, none of the 49 Democrats in the Senate has publicly taken a stand against new protections for DACA recipients. That includes Montana’s Jon Tester, the only Democratic “no” vote from 2010 who remains in the chamber, and West Virginia’s Joe Manchin, who missed the 2010 vote but said he would have voted against the Dream Act at the time.
This is a fairly clear-cut case study in the party's trend leftward. Two points explain this movement: (1) The Democratic base has become more progressive and (2) the Democratic base is newly energized. That means members of Congress are facing intense pressures from a base that's more radical and more active, giving the far Left more power. And these pressures are even starting to influence Red-state Democrats.