Greg Sargent from The Washington Post lays it out clearly in this article:
[The most recent Washington Post/ABC News Poll] showed that all of the top-tier Democratic candidates defeat Trump in head-to-head matchups by sizable margins among all U.S. adults. But among registered voters, those matchups suddenly become mostly dead heats.
Specifically, among all adults, Joe Biden leads Trump by 55 percent to 41 percent; Bernie Sanders leads by 51 percent to 45 percent; Elizabeth Warren leads by 51 percent to 44 percent; Kamala D. Harris leads by 51 percent to 43 percent; and Pete Buttigieg leads by 48 percent to 44 percent.
But among registered voters, Biden leads Trump by 53 percent to 43 percent, while all the others are locked in a dead heat with the president.
Sargent then cites estimates that the voter turnout could rise from 138 million in 2016 to 160 million in 2020. Yes, the turnout in 2020 will be partly caused by increases in voter numbers by younger people, college-educated whites, and minorities. But it may also be driven by greater participation by Trump’s base, non-college educated whites. WE MUST ASSUME THAT TRUMP SUPPORTERS WILL BE OUT IN FULL FORCE IN 2020.
Sargent makes a highly salient observation:
...the disparity in the [poll] matchups among all adults on one hand and among registered voters on the other is a reminder that as a general matter, Democrats will reduce the chances of another fiasco if they do a good job at tapping into the pool of eligible voters. That will, of course, entail registering them and turning them out.
Hmmm. Sounds like an argument someone else I know has been making, in posts like this:
Information on registering voters.
The Sign Ten to Vote Campaign
Get out the damned vote!
Our target groups: WHERE THE VOTES ARE
Reaching out to 100,000,000 voters FACE TO FACE
Where to give money to Trump’s 2020 opponent
Going after Puerto Rican voters in Florida
Several major ideas in one place
A call for party unity
A reminder of what’s at stake
The survival of the United States as a free country depends on a Democratic victory in 2020. The time for action is NOW.
And as I’ve said many, many times before: