Dividing the left wouldn't matter if we used a more representative voting system. One that gave people the freedom to vote how they want and still have their vote count if their preference didn't win. Voters should be able to transfer their vote how they wish and stay represented. To have their vote count no matter what.
Why don't blue states switch away from First-past-the-post voting? Republicans aren't in power, they could easily make this change. Don't they believe in democracy? Or do prefer this undemocratic hostage situation that hands the republicans power repeatedly?
Electoral Reform Videos
First Past The Post voting (What most states use now)
Videos on alternative electoral systems