The chart above shows the results of a recent CNN / SSRS Poll -- done between September 5th and 9th of a national sample of 1,639 adults (including 1,526 registered voters). The margin of error for both is 3.2 points.
This poll didn't ask who the respondents would vote for in the next election. Instead, they asked whether Donald Trump deserved to be re-elected. A clear majority of both adults and registered voters said NO -- adults by a 24 point margin and registered voters by a 19 point margin. And majorities of all the demographic groups agreed (gender, age, race/ethnicity, income, and education).
This doesn't guarantee the Democrats will win the White House in 2020. That will depend on who they nominate and how well they can turn out the vote. But it does mean they have a golden opportunity in 2020.