Rain fell throughout Friday and was expected to linger as the NL East champions began their final series of the regular season.
Washington began the day two games ahead of the Los Angeles Dodgers for home-field advantage in their upcoming matchup in the NL Division Series.
Nationals manager Dusty Baker has given Harper additional time to heal after the reigning NL MVP jammed his thumb on a slide Sunday at Pittsburgh.
"If a guy tells me he's ready, if I have a luxury, I like to give them one more day off," Baker said. "I just noticed that a lot of guys say they're ready, and you bring them back and they're really not ready. I would prefer that you be champing at the bit to get back (instead) of a person to just say that you're ready."
Baker also took precaution with Harper because of the wet conditions in the Washington area this week.
Harper is batting .244 with 24 home runs and 95 RBI.