Washington Nationals' Danny Espinosa swings on a three-run triple during the seventh inning of the Nats’ second win over Milwaukee on Sunday. (Alex Brandon/Associated Press)

Their batting average, as a team, is hovering just above the Mendoza Line. Their splashy, nine-figure free agent owns two RBI. Their slugging percentage could be mistaken for a frat boy’s blood-alcohol reading. Their best player has been a spectator almost half the season. And yet these Washington Nationals, winners once in their first five games, have scraped their way to a winning record.

By sweeping the Milwaukee Brewers before 23,047 at Nationals Park in a straight doubleheader Sunday by scores of 8-4 and 5-1, the Nationals nudged their record to 8-7, above .500 for the first time since May 28 of last year.

They readily admit they have not yet started to hit, and without Ryan Zimmerman in their lineup for another dozen games or so, it may be a while before they do. They are, in the words of Manager Jim Riggleman, doing what they must to “scratch out wins.”

Sunday, that meant more stellar starting pitching and a newfound burst of timely hitting, most of the latter provided by rookie second baseman Danny Espinosa. In Game 1, Espinosa gave the Nationals the lead in the fifth with a three-run home run. In Game 2, when right fielder Jay son Werth sat out for the first time this year, Espinosa broke a 1-1 tie in the seventh with a bases-loaded, bases-clearing triple to left field. The six RBI gave Espinosa a team-high 14 for the season.

The Nationals, who have gone 5-2 with Zimmerman on the disabled list, scored a season-high eight runs in the first game, but they gave their starting pitching credit for the sweep. Nationals starters have upended the preseason conventional wisdom that they would only drag down the rest of the team. Jason Marquis and Livan Hernandez both pitched seven innings Sunday, Marquis allowing two runs, Hernandez one. Together, they walked one batter.

“What’s really impressive is, our offense really hasn’t clicked the way it’s capable of,” first baseman Adam LaRoche said. “Fifteen games is nothing. We’ve got a long way to go and a lot can happen. But if we keep playing like this, it might be a good situation.”

Hernandez and Marquis continued a streak of seven straight quality starts by Nationals starters. In 15 games, Nationals starting pitchers have allowed three runs or less 13 times. Only one team in baseball has received at least five innings every game from its starting pitchers this year, and that team is the Nationals.

“We don’t have the big names,” Hernandez said. “It’s not about who has the most good-looking names. We’re trying hard to be the best and try to give the team a chance. We’ve got a great group.”

The Nationals may have entered Sunday with the worst batting average and lowest slugging percentage in the majors. And while sweeping the Brewers series, they may have used only six hits that drove in runs. But there are 12 major league teams with a winning record, and the Nationals are one.

“We’re grinding it out right now,” said Marquis, who won for the first time at Nationals Park since he joined the Nationals before the 2010 season. “Guys aren’t starting off as hot as we want to, but we’re [above] .500. Once this team starts rolling, we’re going to start playing some great baseball.”

The Nationals busted out in their first game, scoring a season-high eight runs on 11 hits, 10 of them off Yovani Gallardo, one of the most electric young pitchers in the majors. Along with Espinosa’s homer, Ivan Rodriguez crushed an opposite-field, three-run shot — his sixth home run in 524 at-bats — and Ian Desmond added a solo homer in the eighth.

Marquis became the first Nationals starter this season to throw a pitch in the eighth inning. In his third start, Marquis continued to allay concerns that his poor 2010 showing was caused by anything other than the bone chips he eventually had removed from his right elbow. He allowed two runs on nine hits — all singles — and a walk in seven innings, striking out four of the first 10 batters he faced.

Hernandez handled the second game, navigating through seven innings while allowing no walks and six hits. Jerry Hairston, who entered the game 1 for 23 this year, launched a solo home run to the first row of seats behind the left field fence to give the Nationals the lead, his first hit in a game in which he reached base four times. The Brewers tied the game at 1 in the fourth with three consecutive one-out hits, culminating with Yuniesky Betancourt’s single up the middle.

Against former Nationals pitcher Marco Estrada, the Brewers’ No. 5 starter only because of injuries, the Nationals mustered nothing else. Once they got into Milwaukee’s bullpen, they rallied.

In the seventh, Laynce Nix, Desmond and Hairston greeted Kameron Loe with consecutive singles. With Werth standing next to him, Riggleman sent Matt Stairs to pinch-hit. Stairs grounded to second baseman Rickie Weeks, who fired home to prevent the tying run from scoring.

Up came Espinosa. The Nationals’ offense had been buoyed by patience — the Nats entered Sunday tied for third in the majors with 59 walks — and smart baserunning. What they really needed was hits in spots like this — entering Sunday, they had a .628 OPS with runners in scoring position.

“I just needed to put something in play hard,” Espinosa said. “When I try to do too much is when I don’t come out successful.”

Espinosa, emerging as a very early rookie of the year candidate, delivered. Batting left-handed, he took one strike from Loe, then shot a line drive toward the left-field corner. Ryan Braun dove but missed the ball by several feet. All three runners bolted home, and Espinosa slid headfirst into third.

After LaRoche smacked his second home run of the season, a solo shot in the eighth, the Nationals finished with 11 hits in the second game, 22 for the day. They finished the day with a .225 team batting average for the year, up 14 points from the .209 with which they began the double-header.

The stats 15 games into the year signify very little, the sample size still too small to necessarily indicate a larger trend. And the stats, anyway, aren’t what the Nationals are interested in.

As the sun set over the park Sunday, shouts of “Sweep! Sweep!” cascaded from the stands. That is what the Nationals are interested in. They will start play on a road trip Tuesday against the St. Louis Cardinals. They know it will be a challenge. They are also confident both teams will have one.

“We know how good St. Louis is,” LaRoche said. “I think we may have opened their eyes, too.”