Michael Dukakis has seen this script before: a Republican administration besieged by scandal and running out the clock on its second term, while wide-eyed Democrats confidently lick their chops, knowing there’s no way in hell voters will reward the G.O.P. with four more years in the White House. It was around this very moment 20 years ago, the summer when Oliver North told Congress he was “authorized to do everything that I did” and Reagan fatigue took hold, that Mr. Dukakis, then the 53-year-old governor of Massachusetts, emerged at the head of a crowded Democratic presidential pack. By the time...