It's August. We'd be saying hello to President Dukakis and Dewey and Carter would have beaten Reagan, if the election were held in August. Of course, Trump or any major Presidential candidate could overcome a deficit in August.
I won't touch 1948 because I have no idea how accurate polling was overall. However, Carter was not polling over 45% at any point post-convention, nor was Reagan ever behind by 10-15 points (in fact Clinton leads where Carter was at that point by 10-15.)
As far as 1988, Clinton's scenario is actually closer to Bush's than Trump's, as far as good approval ratings for the incumbent party.
I'm having an increasingly difficult time seeing Trump recover from where he's at, but if he does and ends up winning, 2000 would be the closest comparison polling-wise.