Trump needs to do a lot more than criticise the democratic party policies. He must be critical of the culture that spawned them too. He should engage in the culture war. It will be difficult because his entire playboy life history. Many of his strongest challengers will be fringe democrats from the left side who will also be highly critical of the democratic party. They won't beat Trump in 2020 but they could absolutely smoke the next guy, say Mike Pence for example.