Dobbs added an RBI single with two on in the ninth to cap the scoring.
Marlins right-hander Javier Vazquez was effective in four innings, allowing one run while striking out three. But the right-hander suffered the loss.
Espinosa went 3-for-3 and he needed a triple to collect the cycle. in the third inning, he delivered an RBI double to open the scoring. In the fifth inning, he added a two-run homer off lefty Mike Dunn.
2010 Spring Training - null
Sights & Sounds
Spring Training Info
Marquis, meanwhile, scattered two hits and struck out three in four innings. He has now logged seven scoreless innings in his two starts, allowing three hits with one walk and four strikeouts.
"I'm making my pitches," Marquis said. "I'm keeping my sinker, changeup and slider down in the zone. If that happens, the results are going to come."
The Nationals broke the game open with a five-run seventh inning. Espinosa had an RBI single in the inning. Harper came through with a pinch-hit RBI single off Brian Sanches' first pitch.
In his second Grapefruit League start, Vazquez scattered four hits. Two double play ground balls helped the veteran work out of a couple of scoreless innings. Of his 63 pitches, 43 were strikes.
"I accomplished what I hoped with the pitch count today, and I went four innings," Vazquez said. "I felt pretty good out there, and that's the main thing.
"Obviously, we're going to be facing these guys a whole lot during the season. I want to pitch to get them out, but I don't want to show them everything."
Marlins up next: The Marlins have a twin bill of sorts on Thursday, as they are playing day and night games at Roger Dean Stadium. At 1:05 p.m., they will take on the Mets. Anibal Sanchez makes his third Grapefruit League start, and he should throw four to five innings. New York is throwing Dillon Gee. At 7:05 p.m., Florida will face the Tigers, also in Jupiter. Sean West will be making his first Grapefruit League appearance. Justin Verlander gets the start for Detroit.
Nationals up next: The Nationals will split up for a day-night doubleheader in separate spring locales. A 1:05 p.m. ET home game against the Mets will feature Chad Gaudin in the starting slot and Jayson Werth likely returning from mild leg soreness. In the 7:05 p.m. nightcap in Kissimmee, left-hander Tom Gorzelanny will make his spring debut against the Astros after being limited by walking pneumonia. Gorzelanny is competing for the Nats' fifth starting spot.