Why was Dukakis leading Bush by double digits in the summer before the election?

[Fuzzy Bear Loves Christian Missionaries]

Dukakis did a number of things right up through the convention.  He selected Bentsen as his running mate, he ran a non-ideological campaign, and there were Republicans unhappy with the way Bush 41 dispatched Bob Dole in the primaries. 

Several things happened.  The Willie Horton flap, a hypothetical question about mandatory saying the Pledge of Allegiance, and other things opened Dukakis up to attacks on his sense of patriotism and his judgment.  He could have survived these attacks if he had answered them promptly, but he didn't, and by the time public opinion hardened, key swing voters came to believe that the preppy Bush was more of their kind of guy.  Then, too, in a non-ideological election, Dukakis's resume was dwarfed by Bush 41's resume; he had the most impressive resume of any Presidential candidate since Eisenhower in terms of depth of experience. 

Some of the trends that enabled the Democrats to come back from their weakest position in Presidential politics in their history to the point where they could regularly count on entire regions of the country to be their "base" hadn't come to fruition in 1988.  The revolt against the Fundamentalist Christian Right by moderate Republicans hadn't really begun yet; they viewed Bush 41 as one of them.  The nation was not as liberal on racial matters as it is today, and the high profile given to Jesse Jackson by the Democratic establishment was a turnoff to a number of swing voters, many of the nominal Democrats, that Dukakis needed.  The culture war had not yet started, and it was a conflict that most folks would have viewed as a GOP plus in 1988.  People forget that in 1988, it was a big deal if they carried more than DC and one state, and an even bigger deal if a Democrat carried several states in a region.  Put it this way:  Michael Dukakis's 45% was the HIGHEST percentage of the vote a Democrat received (with the exception of Carter's 1976 50% total) from 1968 until 1996, and that includes Clinton's 1992 victory (43%).