The Nationals had never produced six home runs in a game prior to Tuesday's win over the Cubs and now they have done it in back-to-back contests.
Washington won its fourth straight to stay 7 ½ games in front of Atlanta for the lead in the National League East.
Roger Bernadina started the home run onslaught with a solo shot in the second and before the inning was over Harper had hit a solo blast and Adam LaRoche had contributed a two-run affair.
Harper had another in the sixth and both Ian Desmond and Danny Espinosa hit solo shots in the seventh.
LaRoche hit two of the home runs Washington delivered on Tuesday.
Gio Gonzalez (18-7) did not allow a hit until the sixth inning and threw seven shutout innings for the win. He gave up three hits while striking out nine.
Chris Volstad (2-10) surrendered five runs on nine hits with four walks in five innings. It was Chicago's fifth straight loss.