"I'm not one to make excuses," Jackson said. "At the end of the day, you need to get the job done. Today I was just never able to get into a rhythm."
While Jackson only allowed two hits in his other four-plus innings, the big third proved decisive.
Miami starter Carlos Zambrano sparked the four-run frame with a leadoff single, before Jose Reyes followed up with his own base hit. Carlos Lee drove in both runners with a single to tie the game at 2. Two batters later, Jackson surrendered a go-ahead two-run home run to Hanley Ramirez.
It marked the fifth consecutive outing that Jackson gave up a dinger, and the homer was Ramirez's first since July 1.
"This was a total team effort," Marlins manager Ozzie Guillen said. "We got big hits, and that home run by Hanley was huge."
The four-run third quickly turned a two-run Nationals lead into a two-run deficit they were unable to overcome, as they mustered just three runs despite 11 hits and 17 total baserunners.
"That was rough. We had our chances to win the ballgame," Nationals manager Davey Johnson said. "[Jackson] still kept us in the ballgame. We were in position to win that puppy, just one hit here or there. It just wasn't meant to be."
Washington scored the game's first run in the second inning after Jesus Flores drove in Danny Espinosa, who reached on a one-out triple.
Ryan Zimmerman hit a solo shot to left field off Zambrano in the third. It was Zimmerman's 16th career homer against the Marlins. After hitting just five home runs in the first three months of the season, Zimmerman has five in July.
The Nationals cut the Marlins' lead to one run in the sixth, when Zimmerman hit a leadoff double and came around to score on Michael Morse's RBI single. Zimmerman has a seven-game hit streak and finished the night 3-for-3, reaching base in all five plate appearances and coming a triple shy of the cycle.
"Just [have to] keep going, and as long as I feel good, continue to run with it and continue to work and get back to where I want to be," Zimmerman said. "I'm still a long ways away from where I want to be personally."
A day after Washington escaped a handful of jams against Miami, it was the Nationals who couldn't capitalize on multiple chances, stranding 10 men on base and hitting into four double plays.
"[Zambrano] got some good breaks," Johnson said. "Balls hit hard right at people. I thought he was very fortunate."
Washington had a chance to do more damage in the sixth inning with the bases loaded and one out, but Flores grounded into an inning-ending double play.
The Nationals were also unable to convert on chances in the first, fourth and seventh innings. With two on and one out in the first, Adam LaRoche lined into a double play to end the frame.
A similar situation arose in the fourth, when Roger Bernadina grounded into a 4-6-3 double play with two on and no outs before Flores grounded out to end the inning. The Nationals stranded two more in the seventh, while Miami pushed across an insurance run on a suicide squeeze in the bottom of the frame.
Espinosa, needing a home run to complete the cycle, struck out as the tying run in the eighth. With the go-ahead run at the plate in the ninth, LaRoche struck out to end the game and send the Nationals home, where they will host seven straight games against National League East foes.
"We gave ourselves every chance to win that ballgame, that's what good teams do," Johnson said. "Now you go home and get after some other boys. ... I think we're in good shape. The whole lineup is swinging the bat pretty good and the pitchers are throwing the ball good. Bring 'em on."