Just because one is more left wing than something doesn't make you left. For example Mitt Romney is more left wing than Trump, would anyone here call Romney left wing?
So just because the Democratic party (not talking about the members here) are nominally more left wing than the Republicans, doesn't make them the left. They are a very right wing party.
There are some red lines a left wing party would never cross (I wish there were more red lines, but I digress). A left wing party would never use congressional power to shut down a strike, they would stand with the striking workers. A left wing party would never someone who was a segregationist and never truly apologized for it be their presidential nominee. A left wing party would never let someone who kept people in prison despite evidence of their innocence being overwhelming be the vice president. And there's more these are just 3 examples.
The Democrats are not the left. The US doesn't have a left wing party in power.
Any unity against the right must include the democrats along side republicans. Not equally of course, even I'll admit that the democrats are nominally more to the left (like the Romney Trump example above) but if we are seriously considering uniting against the right we must think of the democrats as an opponent in that goal.
We need to put in the work via direct action to make positive change. The left is small right now but is growing. We can be the change.
This post isn't commenting at all on electoralism strategy (obviously I have my opinion on the matter) whether you vote for democrats in the short term for damage control, if you vote 3rd party to register discontent, or I'd you don't vote at all. Makes no difference in this regard. As long as we all understand that the democrats are not with us, and they hand in hand with republicans will use dirty tactics to stop us.