Did they? With the choices provided, it was pick or choose one mistake or the other mistake. The only one's who liked either candidate were their supporters. You had 89% of Democrats vote for Hillary, 88% of Republicans vote for Trump. Normal numbers based on percentages of the history of how each party votes for their candidates. It was independents that gave Trump the Oval Office, not Republicans or Trump supporters. Only 27% of the total electorate identified with the Republican Party in November of 2016 vs. 31% with the Democratic Party. 40% of the electorate as independents. Trump won the independent vote, hence the presidency. Trump wouldn't be president if he hadn't won the independent vote pure and simple.
Most independents in 2016 didn't vote for either Trump or Clinton because they wanted one or the other to win. They voted for the candidate they least wanted to lose. You can see this in the fact Trump was seen negatively by 57% of independents, but Clinton was worse, 70% of independents viewed her unfavorably or negatively. Questions 10 and 11.
https://d25d2506sfb94s.cloudfront.net/cumulus_uploads/document/l37rosbwjp/econTabReport_lv.pdf
Now you aren't going to change Trump's support among the avid Trumpers, just like you weren't going to change Obama supporters or G.W. Bush's or Bill Clinton supporters against them. That's politics, probably partisan party politics. Those you can change are the independents, the swing voters, those who are less or non-partisan, those who don't affiliate with either party. You have seen a change among them. But they aren't avid Trumpers or partisan party supporters.
You seen it in the fact independents went from voting 46-42 in favor of Trump to voting 54-42 for Democratic congressional candidates. You seen it in how independents view Trump, 48% favorable Trump at the end of January 2017 down to 38% today.
Which in my opinion isn't surprising. You had 42% of independents who disliked Trump so much they voted for Hillary. You had another 12% in 2016 who disliked both major party candidates so much they voted third party. You could look at their votes as having 54% of independents against Trump on election day, 58% of independents against Hillary Clinton if you want to use votes instead of favorable/unfavorable stats. Hillary is gone, wasn't on the ballot in 2018. So guess what, her 42% she received plus the 12% who voted third party, all anti Trumpers comes out to 54%. The exact number of independents who voted for Democratic congressional candidates in 2018.
Bottom line, no one is going to change an avid Trumper into an anti-trumper. No one could ever change a pro-Obamaite into an anti-Obamaite. But you can change swing voters, independents. I'd concentrate on them and forget the avid Trumpers. It was independents who put Trump into the White House, it will be independents who defeat him in 2020. That is unless the democrats nominate another Hillary Clinton. Another candidate as much disliked by America as a whole as Trump is.
So the question is, have the Democrats learned from 2016? Will they nominate a candidate that appeals to independents, Hillary didn't. Obama did. Obama won, Hillary lost. Think about it.