It's the Astros.
A point not entirely without validity - three of the five poorest-drawing home matches in Nats history (Nats Park or RFK) occurred in a weeknight series against that team in September 2010:10,999 Monday 20th September 2010 vs Astros
11,191 Wednesday 21st April 2010 vs Rockies
11,623 Monday 19th April 2010 vs Rockies11,893 Tuesday 21st September 2010 vs Astros12,213 Wednesday 22nd September 2010 vs Astros
Also...expect the thin attendance to continue through the rest of the homestand: besides tonight's Astros finale (going up against a Caps home playoff match), Marlins have always been a lousy draw here to begin with, and there's a good chance of rain Saturday and Sunday (50-60% per Capital Weather Gang).