Florida’s Bryan Petersen is mobbed after drilling the game-winning walk-off home run in the bottom of the ninth to guarantee a sub-.500 record for the Nationals, who fall to 79-61 with one game left. (Lynne Sladky/AP)

At this time last year, Jayson Werth played for Charlie Manuel, a manager who possesses a special knack for words. Tuesday afternoon, a Manuelism came to Werth’s mind when he considered the difference between the Washington Nationals, his new team, finishing better than .500 or one or two wins shy.

“A lot means a little,” Werth said. “Charlie used to say that all the time. If we finish over .500 versus a half a game under .500, it would really be a big difference. It really would. I think if we finish with a winning record, that’s quite a statement for this team.”

The Nationals clung to the chance at the team’s first winning record since baseball returned to Washington until the ninth inning Tuesday night, when the Florida Marlins suddenly sealed a 3-2, walk-off victory. With two outs and the score tied in the ninth, Bryan Petersen launched a home run to deep right field. Left-handed reliever Doug Slaten hung his head immediately and only peeked at the ball before it soared over the fence.

Rather than entering the final day of their season playing for their 81st victory, the Nationals dropped to 79-81 and sealed their sixth straight losing record. Since the Nationals had a game against the Los Angeles Dodgers wiped off their schedule by rain this month, they will play 161 games and will not have a chance to equal the 2005 Nationals’ 81-81 record.

The Nationals have still won 13 of their past 17 games, and they have already improved upon their 2010 win total by 10 games. But they will not finish better than .500, and to at least some of them, that means a lot.

“I’m really very proud of them,” Manager Davey Johnson said. “That was a weird event that ended this ballgame. I was comfortable with where we’re at, getting a lot of guys in the game and they were doing well. But no, it’s been a great season. I’m really proud of these guys.”

John Lannan concluded his season by allowing two runs in six innings on three hits and three walks. He finished his year with a career-best 3.70 ERA, but he left Sun Life Stadium thinking more about the pitch that may have cost him his 11th win.

In the top of the sixth, the Nationals took a 2-1 lead when Ian Desmond’s single drove home Steve Lombardozzi, who had reached on a double. In the bottom of the inning, Emilio Bonifacio ripped a leadoff double down the third base line. After Lannan retired the next two hitters, Bonifacio stole third base.

Primed to strand Bonifacio and preserve the lead, Lannan momentarily lost his command. He got two strikes on Gaby Sanchez, then hit him in the foot with a curveball. He then threw three consecutive balls to Logan Morrison, a left-handed hitter.

Lannan came back with a strike, then fired an 88-mph fastball that catcher Wilson Ramos had positioned himself to catch low in the strike zone. Lannan sailed it over Ramos’s head, still within his reach. The ball skidded off Ramos’s glove and trickled behind him.

“There was a couple pitches there where I think flew open or something happened,” Lannan said. “I don’t know, but I was missing huge. It’s not easy back there when I’m missing my spot by five feet.”

Bonifacio, one of the fastest players in baseball, bolted for home. Ramos scampered to retrieve the ball as Lannan dashed to cover the plate. Bonifacio, Lannan and the ball arrived at roughly the same time, but Bonifacio beat the tag and tied the score. Hours later, Lannan still harshly blamed himself.

“I kept the team in the ballgame,” Lannan said. “But you know, you look at that, and I could have easily just kept the lead. They’re busting their butts out there to get a lead, and when you blow it like that, it’s not a good feeling. Going into next year, that’s something I really want to work on. When guys get the lead, hold it.”

The Nationals mustered no more offense against Marlins starter Javier Vazquez, who made what will probably be the final start of his career. Vazquez yielded five hits and struck out nine in a complete game.

The one Nationals hitter who touched Vazquez was Michael Morse. In the fifth, Morse reached down for a 90-mph sinker at his ankles and slammed his 31st home run of the year to the first row in right-center field.

Afterward, Johnson said he planned to rest Morse for Wednesday’s finale because “he’s a little worn out.” The actual reason may lie with Morse’s .303 batting average. By not playing Morse, Johnson would assure Morse’s ending the year a .300 hitter, an accomplishment for a slugger who this season spent no time in the minors for the first time in his career.

“It doesn’t matter to me,” Morse said. “I’m trying to win games.”

If the Nationals do end their season with a win, it will bring them only to the cusp of a winning record. Slaten entered in the ninth and recorded two quick outs. With the second pitch he threw to Petersen, though, there was never a doubt.

Losses for the Nationals have been rare this month, but Tuesday night’s, and the opportunity it erased, stung a little more. Players had talked among themselves about finishing above .500 in recent days, and the clubhouse felt more sullen than after a typical playing-out-the-string defeat.

“I didn’t want to end like this,” Lannan said. “Move on, we got one more game tomorrow.”