From left to right, David Lough, Nick Markakis and Adam Jones celebrate Baltimore’s 7-3 victory over the Nationals. (Patrick Smith/Getty Images)

The lead and the game were already slipping away from the Washington Nationals when Craig Stammen took the mound in the seventh inning Monday night. Moments earlier, starter Tanner Roark turned a one-run lead into a one-run deficit against Baltimore. With one out and a runner on first, Stammen, a groundball pitcher, was looking to induce a double play.

Instead, Manny Machado and Adam Jones laced singles. The lead grew to two runs and chants of “Let’s go O’s” rained down in Nationals Park. The Orioles continued to hit Stammen in the eighth — he yielded six hits in his one inning of work — and Baltimore pulled away for a 7-3 win.

After a recent stretch of strong starts, Roark had a subpar performance, allowing five runs, three in the decisive seventh inning. The bullpen didn’t help, but neither did the Nationals’ offense, which managed just three runs out of 10 base runners. The loss trimmed their division lead over the idle Atlanta Braves to three games.

“Whenever you go into the game, you’re trying to keep the game close,” Stammen said. “Or try to keep the lead. That didn’t happen [Monday] so it’s disappointing.”

The schedule initially had the Nationals and Orioles enjoying a day off Monday. A July 8 rainout changed that, and Monday’s win gave the Orioles three wins in four games against their National League neighbor.

Entering the game, Roark had given up only four runs over his previous 28 innings. Against the Orioles, who have hit the second most home runs in baseball, Roark wasn’t as sharp. Mistakes against them are clobbered over the wall.

Roark was staked to a 1-0 lead after Wilson Ramos’s home run in the bottom of the second. He gave it right back in the third. He made a mistake to light-hitting catcher Caleb Joseph by leaving a fastball over the plate and the Orioles’ No. 8 hitter crushed it about five rows into the left field seats to draw Baltimore even.

The Nationals took a 2-1 lead in the inning when Jayson Werth swung at a 3-0 pitch with two on and hit a sacrifice fly to drive in Denard Span. But the Nationals couldn’t push across any more runs in the frame.

“That one was potentially setting up for a big one but that’s the way the game goes,” Manager Matt Williams said.

Ramos’s double-play grounder in the fourth gave Roark a 3-1 lead.

In the fifth, Roark missed again with his location, this time to the dangerous Nick Markakis. Ramos set up outside but Roark’s curveball hung over the heart of the plate. Markakis pulled it but it stayed a few feet fair, landing in the far right corner of the Nationals bullpen. The lead was trimmed to 3-2.

“They are in first place for a reason,” Roark said. “They can hit the ball and it showed.”

Williams made his decision to let Roark pitch the seventh an inning earlier when the pitcher came up with two men on with two outs in the sixth. Roark’s pitch count stood at 84, and he has shown he can handle the bat. But Roark hit a sharp grounder to first baseman Chris Davis, who scooped it up to end the inning.

“He almost got a hit,” Williams said. “That ball almost got down the line. But in that situation, he’s low pitch count, he’s pitching well. He’s been pitching well. So we want to take the lead into the following inning, as well. And we can’t burn the pen, either. Decided to go with him the last inning.”

Williams said the decision to leave Roark in also depended on who was available in the bullpen. Often with a lead, Williams has turned to Drew Storen in the seventh inning, Tyler Clippard in the eighth and Rafael Soriano in the ninth.

Roark certainly earned the right to pitch the seventh, as he has in his previous five starts. Williams said the bullpen has been “pitching a lot lately.” Storen hasn’t pitched since July 30 and, after the game, said he was available if needed. Clippard pitched on Sunday.

Asked who was unavailable, Williams offered a vague answer. “That’s for us to know,” he said. “I’m not gonna go there now. But the bullpen’s been pitching a lot.”

The Nationals’ bullpen has been among the best in baseball all season but has hit a rough patch of late. Over the past 15 days, it has allowed 19 runs over 331 / 3 innings.

“I don’t think we’re wearing down,” Stammen said. “We’ve proven, I think all of us, that we can withstand major league seasons and that we’ve all pitched well in the season. Even though this is a little lull, maybe the averages are just evening up a little bit and we’ll get back on track here shortly.”

The seventh would be Roark’s undoing. He gave up a hard groundball into the left field corner to J.J. Hardy for a double. He then left a fastball up for up for Ryan Flaherty, who drilled a double off the center field wall to tie the game at 3.

Pitching coach Steve McCatty came out. Roark responded by getting his first out of the inning, a sacrifice bunt by Joseph. Pinch hitter Delmon Young then singled to center to give the Orioles a 4-3 lead, ending Roark’s night.

“The ball was up, elevated, wasn’t hitting my spots,” Roark said. “. . . If the ball is up and I’m not hitting my spots, the ball is going to be hit hard. That’s what happened.”

Ross Detwiler gave up a single to Markakis. Stammen, who has pitched only twice in the past 13 days, came in but the Orioles weren’t done. Manny Machado singled to load the bases and Jones piled on with another hit, making it 5-3. By the time Stammen was lifted, Baltimore had a four-run lead and he had given up six singles. And the Orioles chants continued.