The way he's been going recently at home, Newcomb is leaving doubt that he can handle the job.
"It's just more a mentality," he said. "I've got to go out there and get right after guys and not worry about too much about what's going on as far as what's working and what's not."
Atlanta maintained a 6 1/2-games lead in the NL East over second-place Philadelphia but has lost two straight following a season-best, six-game winning streak. The Braves are 38-36 at home, where they have lost eight of their last 11.
Newcomb (12-9) made 31 pitches in the first and allowed five runs and six hits in three innings. He has lost four of five home starts since coming within one strike of a no-hitter against the Los Angeles Dodgers in Atlanta on July 29, and he has an 11.32 home ERA over that span.
"I wasn't commanding the fastball, so it was hard to get everything else going," Newcomb said. "You could tell they were laying off pitches, waiting on their pitch to kind of do something with."
Washington built a 3-0 lead in the first when Harper hit his 34th homer and Ryan Zimmerman's grounded into a run-scoring forceout. Rendon hit his 20th homer in the third to make it 5-0; he has reached safely in 25 straight games.
Soto singled in the first to extend his on-base streak to 21 straight games, tying Mickey Mantle in 1951 for the second-longest by a teenager behind Mel Ott's 22 in 1928. Soto is batting .357 during a 10-game hitting streak.
"He's been a blessing for us all year long," Nationals manager Dave Martinez said. "Just watching him play, he's just out there having fun and playing the game the right way."
Roark's streak of 27 2/3 consecutive innings without a walk ended in the third, and he struck out Freddie Freeman to strand two runners.
Rendon dove to field a grounder at third in the fifth and jumped to throw off one leg to get pinch-hitter Preston Tucker at first. Late in the inning, center fielder Victor Robles ran hard to his left and chase down Ozzie Albies' liner at the wall.
But it was the Nationals' offense that dominated the last two games. Harper just hopes Washington can stay in contention despite being 7 1/2 games back of Atlanta.
"It's a bummer when we're not winning ballgames," Harper said. "That was the biggest thing on my mind. I'm not really worried about my numbers or anything like that because I am who I am. That sounds bad, but I am. Any given night I'm able to go out there and do some things for this team that are special, and I was able to do that again today."
Braves 3B Johan Camargo missed his fourth straight game with left groin tightness, but manager Brian Snitker said he should be back in the lineup on Monday. ... Atlanta RHP Arodys Vizcaino was not available after pitching a scoreless ninth inning Saturday, the former closer's first appearance after he missed 55 games with right shoulder inflammation.
RHP Erick Fedde (2-3, 5.12 ERA) will make his first career appearance against the Marlins when the Nationals begin a two-game series Monday at Miami. RHP Mike Foltynewicz (11-9, 2.66 ERA) will face 15-game winner Miles Mikolas when the Braves begin a three-game series against St. Louis.
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