If that power is 'waning' so much, why have "Democrats have become the opposition party, holding a minority of seats in both the House of Representatives and the Senate. The party also holds a minority of governorships (15/50) and state legislatures (full control of 12/50, split control of six others)"

And yet again you just cite an "I don't like Trump because" piece with zero positive suggestions as to who or what policies will/can/should replace him that can actually get elected.

How will the Democrats increase their Governorships, their State Legislative seats, their seats in the House and Senate?

The DNC and Hillary played the 'Trump is evil' card and got trumped.

The Democrats hoped and spent millions on a couple of races so that the first four Congressional openings would repudiate him.  Didn't happen.  

Oregon is what it is.  

How will the Democrats win all those seats back in Red 'Flyover Country?