The charts above are from a recently released Gallup Poll -- done between July 1st and September 30th of a national sample of 12,332 adults, with a 1 point margin of error.
It shows that the United States has 5 points more Democrats than Republicans (47% to 42%). At this point in 2015, the margin was only 2 points (45% to 43%). But Trump has driven some Republicans out of the GOP, and some Independents to the Democrats.
This doesn't necessarily mean the Democrats will win in 2020, but it's a good sign. We know the Republicans will get most of their 42% to the polls next November. The Democrats need to do the same with their 47% -- and that could easily happen, since Democrats are currently highly energized.
If the Democratic turnout is strong, they will only need to split the 11% of true Independents. That can be done, considering the continued unpopularity of Trump.