After giving up a solo home run to Jesus Flores to lead off the second, a single to opposing starter Edwin Jackson and two straight doubles, Rusin was pulled after facing only four batters in the frame.
Altogether, the 25-year-old left-hander surrendered five runs on eight hits and two home runs over one-plus inning of work.
"Without breaking balls and stuff, you better keep it down and you better have movement," Cubs manager Dale Sveum said. "Today, he didn't have either. The cutter wasn't doing a whole lot, and then he couldn't get the ball to sink or keep the ball down, either. He threw a few good changeups; that was about it. It's tough to get through a Major League lineup -- let alone that lineup -- with a flat fastball."
Jackson, meanwhile, held the Cubs to four runs over his 5 2/3 innings, striking out eight.
After Sveum pulled Rusin, Jaye Chapman made his Major League debut in the second. Also called up earlier in the day, the 25-year-old right-hander gave up only a walk in the frame. Chapman was pulled for pinch-hitter Tony Campana in the third.
Sveum proceeded to substitute pitchers regularly, having rookie Blake Parker pitch the third -- he gave up two hits, but zero runs -- and Rafael Dolis take the fourth. Dolis, who was also called up from Tennessee earlier in the day, got two outs before unraveling, giving up a solo blast to Adam LaRoche, a double to Danny Espinosa and then another homer to Tyler Moore.
"We are in a pretty good place right now, offensively," Nats manager Davey Johnson said. "We have been kind of building to it. I like the lineup that we have; we haven't had this lineup very long."
Dolis was briefly the Cubs' closer earlier this season, but he went 2-4 with four saves and a 6.44 ERA in 29 1/3 innings pitched in 27 games.
"He threw strikes," Sveum said. "That was good, at least he came back. Hopefully that builds on something, because when he left, he had trouble. At least, hopefully, he has a little confidence. The velocity was good again, but location and the movement still isn't there. But at least he threw strikes."
Miguel Socolovich handled the next two innings and allowed two hits, including a solo shot by Ryan Zimmerman in the sixth.
"Even though we gave up a lot of runs, at least the bullpen will be all right," Sveum said. "They gave it up, but didn't give up too many pitches. That's probably the only bright spot of the day, really."
Jackson limited the Cubs to one hit through the first three innings before Chicago got on the scoreboard in the fourth. Luis Valbuena and Anthony Rizzo each singled before an Alfonso Soriano double play advanced Valbuena to third. A Starlin Castro single scored Valbuena for the Cubs' first run, but Castro was picked off after one pitch to Welington Castillo.
The Cubs added three more runs in the sixth, all with two outs. Valbuena singled again and Rizzo walked before Soriano knocked a triple high off the wall in left-center, inches away from a home run. Castro followed with another single to score Soriano before right-hander Christian Garcia -- also making his MLB debut for Washington -- forced Castillo into an inning-ending popup.
Alberto Cabrera entered for the seventh, allowing a double to Bryce Harper and then an RBI-single to Jayson Werth, and LaRoche's second homer of the game came in the eighth off Jeff Beliveau.
Anthony Recker, one of the four Cubs callups earlier in the day, hit for Beliveau in the ninth, flying out to right field. In doing so, he became the 52nd player used by the Cubs this season -- breaking a franchise record.
"Well, you'd rather have it another way," Sveum said. "There's probably nothing good when you break a franchise record for players used. That's usually not going to end up [happening] in a good season."