"I thought," Braves manager Fredi Gonzalez said, "that the division was going to be a lot closer."
Upton led off the eighth inning with his 20th homer, and Atlanta extended its winning streak to a season-high 11 games by beating fading Washington 3-2.
"You want to step on their necks, especially when we've got this big of a lead," said Freddie Freeman, who drove in Atlanta's first two runs with a pair of singles off Stephen Strasburg in the third and fifth. "We can expand it a little bit more against them. I think we set the tone tonight."
Reliever David Carpenter (3-0) earned the win by retiring all five batters he faced after Mike Minor went six-plus innings, giving up eight hits and three walks, yet allowing only two runs. With Braves closer Craig Kimbrel getting a night off after pitching in the previous three games, Jordan Walden earned his first save of the season, but not without some difficulty.
Walden gave up a leadoff single to Anthony Rendon, who went to second on Denard Span's sacrifice bunt - although Nationals manager Davey Johnson wanted Span to try to bunt for a hit there. Rendon took third with one out on Walden's wild pitch, giving Washington a terrific opportunity to tie the score.
But clutch hitting was the Nationals' biggest problem on this night - and all season, really - and Scott Hairston popped out to the catcher in foul territory, before pinch-hitter Chad Tracy flied out to left.
All part of Washington's 1-for-10 performance with runners in scoring position.
"Came up with nada," Johnson said.
The Nationals again failed to provide much run support for Strasburg, who struck out nine in seven innings, allowing two runs and five hits.
The past five times he's allowed two earned runs or fewer, Washington lost.
In every inning from the second through seventh, the Nationals put the leadoff batter on base, but had little to show for it.
The Braves, meanwhile, did a lot with a little. In the third, they followed two infield hits with Freeman's blooper on a 3-0 pitch.
"Lucky golf swing," Freeman called it.
In the fifth, Upton got a two-out single and came home on Freeman's single after stealing second, a recurring issue for Strasburg.
"He has the same pattern every time," Johnson lamented. "He's very quick to the plate, but he is locked in his ways."
Upton, who went 3 for 4, delivered his 20th homer on a 3-2 changeup from Tyler Clippard (6-2), who replaced Strasburg to begin the eighth.
"Not a good pitch," Clippard said.
Shaking his head, along with a plastic water bottle in his right hand, Clippard summed up where things stand for the Nationals at the moment, saying: "We've got to turn this thing around. If it's going to happen, it's got to happen soon. ... Two months left, and we're 13 games back. So we've got to get moving."
Washington led the majors with 98 wins in 2012. But Atlanta's 12½-game cushion entering Monday was the largest margin through Aug. 4 in any division since 2005, when the Chicago White Sox led the AL Central by 14 games, according to STATS.