Turnout patterns, the strength of a campaigns GOTV operation and polling methodology change each election and are all responsible for systematic polling errors.
I am confident that the first two factors you mentioned will be better for us, and the third factor quite possibly could still be better for us. Marquette was right on in 2018 but Trump is a much stronger candidate than Leah Vukmir and Scott Walker.
1. In 2016 you had a decent sized drop off with African American turnout which helped Trump win. Further many voters who did not like wither Hillary or Trump voted third party which allowed Trump to get in with a small plurality of the vote. In his loss Scott Walker got a higher % of the vote than Trump did in 2016. Unlike in 2016 where Democrats did not focus as much time as the needed to on the Midwest you are going to see a strong full fledged GOTV effort in WI this year which should help with Turnout. On top of this your going to see a strong anyone but Trump movement going on which will help drive 2016 third party voters to vote for the Dem candidate.
2.I will concede that many of Trumps supporters from last time will be enthusiastic to go out and vote for him this time but unless he is going to expand his base than it's going to be extremely difficult for him to win re-election this time.
3. Scott Walker has won a higher % of the vote than Trump got in 2016 in every statewide general election he has been in so I fail to see how Trump is a better candidate than Walker is.
