If Donald Trump keeps every last 2016 vote he got in 2020 in each state (nobody has died, nobody who voted GOP in 2016 votes Democratic in 2020, nobody who voted GOP in 2016 stays home in 2020) ...
... the Democratic presidential nominee can beat him by picking up as few as 77,747 more votes in 2020 than Clinton got in 2016, in three states (Michigan, Pennsylvania, and Wisconsin) with a combined population of 28.5 million.
That's a little more than a quarter of one percent of those populations.
In 2016, voter turnout nationwide (and in Wisconsin) hit a 20-year low (in Pennsylvania, it remained at 2012 levels, in Michigan up just a hair). Republicans were energized; the turnout deficit came out of Clinton's hide.
All the Democrats have to do to win in 2020 is a better job than Hillary Clinton of enthusing a handful of Democratic-leaning voters in three states.
Could they blow that simple job? Sure. Is it likely? No.