"Mr. Biden is far better known than Mr. Dukakis was and he has shown a resilience to caricature that Mr. Dukakis did not have."

"Mr. Trump is viewed unfavorably by a big swath of voters. His lack of credibility with many Americans has undercut his ability to deliver an attack. The nation is more pessimistic than it was when Mr. Dukakis faced Mr. Bush, who as Ronald Reagan’s vice president was effectively running as an incumbent.... 'This is going to be tricky for them: Biden is a pretty well-known quantity,' said Susan Estrich, who was Mr. Dukakis’ campaign manager. 'The way you usually burst balloons is paint the other guy as a risk.' Mr. Dukakis, proud and disdainful of politics, refused to believe these kind of attacks would hurt them, and did not heed the advice of his staff that he fight back. He allowed Mr. Bush to define him before Labor Day.... His opponents even raised questions about Mr. Dukakis’s mental fitness, decades before Mr. Biden faced the same. Conservative groups were circulating rumors, with no substantiation, that Mr. Dukakis was hiding the fact that he had been treated for depression. As the summer came to an end, Mr. Reagan was asked if Mr. Dukakis should release his medical records. 'Look, I’m not going to pick on an invalid,' he said. Mr. Reagan later said this was a failed joke, but by design or not, it succeeded in thrusting the rumor to the center of public attention. Mr. Dukakis called a news conference to say he had never struggled with mental illness."

From "A Glimmer of Hope for Trump? How Bush Mounted a Comeback in 1988/For Biden, a cautionary tale. For Trump, a search for his own Willie Horton" (NYT).