These were people that are mostly independent or lean right. People that were too embarrassed to admit to voting for Trump. There was the other problem that a large number of people voted the first time for Obama. These people didn't represent new liberal voters. They simply stopped voting once Obama was gone.
Both these groups share the same problem. They don't understand how the government works. People that thought Obama and Clinton didn't do enough for them, but don't understand handing the house and senate to Republicans was why Obama didn't do more etc...
The question is will democrats face a double whammy again in 2020. Will Democratic voters stay home (or be kept home by voter suppression), will they lose all the first time liberal voters from 2016? Will Trump gain that silent independent vote too afraid to tell the pollsters they like Trump?
The odds at this point seem unlikely. The early voting boxes by me were mobbed nearly a month before Nov 3rd. People couldn't wait to vote this year. I assume because passions are high. That means you're likely to draw in those never voted people back that faded away after Obama. Voting is the popular thing to do right now. You see this effect when you hear that Shaq voted for the first time ever.
I also see a lot of my independent friends not breaking for Trump. They won't go Biden, but I see a strong surge for libertarians this year instead of Trump. That might cost him Florida and Ohio. I think that it's this last turnout that worries the GOP. This is the turnout that put them in the front seat in the House, Senate and presidency in 2016. But is there really a silent embarrassed Trump fan out there in 2020 like 2016? I just don't see it. Being pro-Trump is the shits for RW people. He's no longer your secret passion. This time the polls are probably capturing Trump's real support. It's why Biden is so confident and Trump looks like a trainwreck. His internal polling is telling him, those independents aren't there this year. They in fact hate him.