While this is a little bit of poll cherrypicking, I do think Dems are overrating the chances of a colossal wave that means they can run as far left as they like. I suspect that part of what's preventing them from running away with it is the perception the party is too left.
The Dems midterm warning sign is pretty clear:
There are high quality polls (Ipsos, MU Law, & NBC), which show Dems shy of their generic ballot benchmark in past wave years.
Their Dems avg. lead is just 3 pts... & Dems need a 3-4 pt win to take the House given redistricting.