It’s pretty simple. You do what Hillary didn’t: have an economic message that wins back Obama voters who defected in ‘16.
A centrist neoliberal message won’t cut it this time. You need to offer what Pubs will not, a fucking hand up. As I said above you also need to get activists wet for the candidate. The best path there is with a candidate like KH who pairs hints of a lefty populism (critical of TPP for lacking worker protections) with the obvious ceiling-breaking GOTV-worker appeal.
Clinton didn’t lose because of racism or sexism. She lost because of many other factors, and one of them was she forgot to fondle the giblets of all of her base. You want to convince bubba in the Midwest that the wall won’t fix their Chevy or keep the lights on? Then offer some real redistribution from the billionaires raping middle America by shutting down mom and pop shops and outsourcing/automating their jobs away. You want to re-convince them that free trade makes more people better off? Give them a reason for the buy-in.
Simply putting up the old white guy won’t fully realize Dem potential in ‘20. He may draw a few white former union workers back, but the base will be deflated. He may win, but it will be closer than necessary with him.