Perhaps the question is, what are the voters looking for?
What were they looking for in 2016?
What are they looking for now?
What won Obama the election in 2008?
I would add anger or disappointment if that works. The recession, continuous wars. Yes, voters were ready for a change, also angry at the Republicans which was first seen in 2006 when the Democrats took back the House and senate.
2016, the voters did change who controlled the house and senate already. I would say 2010 was an anger year for Obama and company not listening to the voters. 2014, so many close elections in which the GOP won 5 of 7. Also the Democrats were defending 21 senators to 15 for the GOP. Also traditional Republican states who had elected Democrats six years earlier went back to their traditional roots, Arkansas, Alaska, Louisiana, Montana, North Carolina, South Dakota, West Virginia.
2020, change maybe. But is it change for change sake or are the voters mad at something, someone? I don't think the voters were mad at Obama or even the Democrats in 2016. Just utterly disappointed in their choices. Two candidate most didn't want.
Are the voters mad at Trump? The Democrats sure are, but what about independents? The ones who gave Trump the white house to begin with? As of today 38% of independents view Trump favorably, 52% unfavorably. When Trump won the independent vote in 2016, independents viewed him 40% favorably, 57% unfavorably. Really no change there. So perhaps it's all about how independents view who the Democrats nominate as to how they will vote. In 2016 Hillary was viewed 27% favorably, 70% unfavorably by independents.
Whoever is the Democratic nominee, I sure think would have better numbers than Hillary. But I've been wrong before.