Bush led in March, April, September, October and November
Dukakis led in May, June, July and August.
So, Bush led the first 2 months, Dukakis led in late spring thru summer, then Bush was in the driver's seat from September onwards.
Dukakis had a solid 4 months in the middle.
So I do not see much of a comparison to 1988, unless you are saying Sanders' is leading now, will trail in late spring into summer, then take the lead for good in September.
The Willie Horton Attacks started on Dukakis in June apparently. But Dukakis led in the polls until September.
Republicans eagerly picked up the Horton issue after Dukakis won the Democratic nomination. In June 1988, Republican candidate George H. W. Bush seized on the Horton case, bringing it up repeatedly in campaign speeches. Bush's campaign manager Lee Atwater said, "By the time we're finished, they're going to wonder whether Willie Horton is Dukakis' running mate."[12]
Seriously, 1988 does not seem to correlate well to 2020.
https://en.wikipedia.org/wiki/Historical_polling_for_United_States_presidential_elections#1972_United_States_presidential_election
I understand people want to raise an electability issue about Sanders, but I just don't see past campaigns from decades ago being particularly relevant.
Sure painting Dukakis as a liberal finally worked in 1988, but the Republicans tried to paint EVERY Democrat as a liberal and they often failed to win.
2020 will have to be judged by it's own unique setting in time and place.