Miami’s Giancarlo Stanton is greeted by Derek Dietrich after blasting a home run off Washington’s Tanner Roark in the third inning. Stanton has 21 homers in 61 career games against the Nationals. (Doug Kapustin/For The Washington Post)

The Washington Nationals’ offense has stalled this month, grinding almost to a screeching halt. The missing production of Bryce Harper and Ryan Zimmerman clearly hasn’t helped, but the level of futility, even for the current lineup, has been head-scratching. Entering Monday’s game against the Miami Marlins, only the Cincinnati Reds had scored fewer runs in May than the Nationals.

On Monday, Adam LaRoche smashed a two-run homer in his second game since returning from the disabled list, but he alone couldn’t save the Nationals in a 3-2 loss at home. The offense couldn’t compensate for an up-and-down start by Tanner Roark or offset Nationals-killer Giancarlo Stanton, and Washington fell under .500 for the second time this season. The Nationals have dropped five of their past six and are closer to last place in the National League East than first place.

“We need to make some runs,” catcher Wilson Ramos said. “If you want to win, we need to make some runs.”

After only three hits Monday, including LaRoche’s upper-deck blast in the sixth inning, the numbers on the Nationals’ May offensive stumbles remain gruesome. They have scored 72 runs in 23 games, an average of just more than three runs per game. They have been outscored 83-72. They are hitting a major league-worst .223 and have a .289 on-base percentage that is second worst in the majors. It has all led to a 9-14 record since May 1.

In April, when the Nationals went 15-12, they scored the seventh-most runs in baseball, averaging 4.3 runs per game.

“When guys are rolling, things are going good and everybody’s hitting and you’re putting up a ton of runs,” LaRoche said. “As a team, when guys struggle, it has a tendency to be contagious at times.”

The offensive struggles have lessened the margin for error for Nationals pitchers. Roark’s handful of mistakes were magnified on Monday.

Before Roark took the mound against Miami, he had allowed only four runs over 46 career innings in Washington. Against the Marlins, a productive offensive team, Roark’s home dominance hit a snag.

He gave up a line-drive double to Stanton in the first inning, which Jayson Werth couldn’t reach in right field, and then an RBI single to Casey McGehee. But Roark’s biggest mistake came in the third.

Ahead in the count, Roark missed inside and hit Derek Dietrich with a pitch. With a runner on base, Roark needed to be careful pitching to Stanton, who has hit 21 home runs and driven in 50 runs in 61 career games against the Nationals. At Nationals Park alone, Stanton entered with 14 home runs and 27 RBI, and an on-base-plus-slugging percentage of 1.139.

On the first pitch to Stanton, Ramos set up his glove behind the plate low and on the outer half of the plate. Roark’s slider didn’t miss by much, as he left the pitch only a little bit higher than he wanted it.

But Stanton’s long arms reached the pitch, and his quick, powerful stroke drove the ball to deep, straightaway center field. The ball hit the wall beyond the center field wall, measured as a 447-foot blast.

“He’s very aggressive,” Roark said. “He’s got that long reach, and I just left it literally down the middle. Mistakes get hit hard, especially with him.”

Added Ramos, “We’ve got to be careful with that guy.”

For the rest of the game, Roark was strong. He gave the Nationals a chance to stay close by firing four more scoreless innings. But the offense couldn’t take advantage.

Against hard-throwing Marlins starter Nathan Eovaldi, the Nationals had only one hit through five innings — Ramos’s double in the second. In the sixth, Werth ended that drought with a single to center field. Then LaRoche, the next hitter, connected on Eovaldi’s low 95-mph fastball and sent the ball into the second deck in right field.

But Eovaldi and three relievers allowed only one more base runner the rest of the way — Eovaldi hit Nate McLouth with one out in the seventh — and the Nationals were left looking for answers.

“Whether it’s pressing and trying too hard or just running into some really good pitchers, I’ve got a feeling it’s a little bit of both,” LaRoche said. “Not putting a big inning together [Monday], we should have done better than that.”