Bryce Harper is off to a flashy start in his first week of action at Nationals Park, but attendance continues to lag behind the team’s hot start. (John McDonnell/THE WASHINGTON POST)

Stephen Strasburg, the National League’s pitcher of the month for April, will take the mound for the Washington Nationals at Nationals Park on Friday evening. In the outfield will be Bryce Harper, the 19-year-old who made his major league debut less than a week ago and has already blown away fans and scouts alike with his arm, bat and swagger.

The two young stars — perhaps the best pitching and hitting prospects in baseball — will be on a District diamond simultaneously for the first time.

And in the stands will be: Who, exactly?

The Washington debut of the Strasburg-Harper duo could scarcely come at a more interesting time. In town will be the Philadelphia Phillies, who not only have won the Nationals’ division for the past five seasons, but whose fans have made it a habit of invading Nationals Park, making it Philly South.

This weekend, though, the Nationals are in first place and trying to reclaim their own territory. The series is being played under a “Take Back the Park” marketing campaign started by the Nationals with aims at actively keeping out Phillies fans. District Mayor Vincent Gray declared this to be “Natitude Weekend” in Washington, playing off the club’s slogan for the year. On Sunday night, the Nationals will appear on ESPN’s marquee “Sunday Night Baseball” for the first time since the opening night of Nationals Park in 2008.

Nationals attendance so far this season has not matched the team’s hot start on the field. (The Washington Post/Major League Baseball)

“I’ve said it before,” Nationals chief operating officer Andrew Feffer said. “I think this is an important moment for our franchise.”

The Washington sports market has long been dominated by the Redskins, who last week drafted quarterback Robert Griffin III and made him an instant celebrity in town. But the quick arrival of Harper, the No. 1 overall choice in the 2010 draft, to join Strasburg, the top overall pick the previous year, would seem to give the Nationals an opportunity to finally create a little buzz of their own.

Yet Tuesday, in Harper’s first game in Washington, 22,675 fans came to 41,000-seat Nationals Park. The following night, with hockey’s Capitals hosting a playoff game less than 21 / 2 miles away at Verizon Center, only 16,274 fans watched Harper rip three hits in a win over Arizona. When Strasburg made his first home start of 2012, 16,245 came to see it — more than 5,000 fewer than had witnessed any of his previous home starts.

Attendance at Thursday night’s game against Arizona was 19,656.

Feffer cites a well-worn list of obstacles to selling mid-week tickets early in a baseball season: iffy weather, school still in session, more competition from other sports. And the club still leans on the fact that it is still, relatively, a nascent entity. The Phillies have played, uninterrupted, in Philadelphia since 1883. The Nationals relocated from Montreal in 2005, becoming the District’s first baseball team in 34 years.

“There’s a whole generation that missed baseball for 30 years,” Feffer said. “It’s still a start-up. The biggest challenge, and probably the greatest opportunity, is building the fan base from the ground up. That takes time.”

Now in their eighth season, the Nationals have never drawn more fans than in their debut year of 2005, when 33,728 fans per game came to outdated RFK Stadium.

Feffer said that although the team’s attendance is down — entering Thursday night, the club was averaging 23,839 fans through 12 home games (21st in the major leagues), as opposed to 24,877 for all of last year — advance sales are up nearly 20 percent.

“I absolutely thought we’d see more [fans] this week,” shortstop Ian Desmond said. “I thought when we were in first place, playing great baseball early on, they’d start coming out. I thought after the offseason acquisitions that people would start coming out. It’s obviously something we have to work on as an organization. We’ve got to play better as players and continue to show them it’s not a fluke.”

By some other measures, the Nationals are already trending upward. Bill Nielsen, the vice president of sales for Scarborough Sports Marketing, which tracks industry trends, said that in 2006 — with data from the previous season — 32 percent of the Washington area’s 4.8 million residents had either attended, watched or listened to a Nationals game. That number bottomed out at 26 percent in 2007. The firm’s latest research, which doesn’t include the 2012 season, has the figure at 33 percent. In 2006, 14 percent of the market attended at least one Nationals game. Now, that figure is at 18 percent.

“It’s kind of what you would expect: an initial splash, then a drop-off,” Nielsen said. “But there has been a consistent build back up.”

Last August, with the Phillies in town, Feffer bristled as boos rained down on the home team and Philadelphia hats and jerseys dominated the crowd. Over the winter, the Nationals — whose former president, Stan Kasten, once wooed Philadelphia fans in an effort to boost attendance — began to block sales to buyers with Philadelphia-area zip codes.

“We had to have a different day here at Nationals Park,” Feffer said.

Still, Philadelphia fans are planning to come. “We kind of laugh at it,” said Kyle Scott, the founder and editor of the Philadelphia sports blog “Crossing Broad” who has organized a bus trip, with close to 200 fans, for Sunday night’s game. “We’re going to come down there and take over the park one way or another. They’re not going to stop us.”

The Nationals, and their fans, seem hopeful — and realistic — about what to expect this weekend.

“I really hope this weekend we’ll have more of our fans than Philly fans for the first time ever,” said third baseman Ryan Zimmerman, the team’s longest-tenured player.

“I think people are excited about it for sure,” said Chris Howdeshell, a 29-year-old Fairfax resident who has been to four games this season. “It’s just getting down there on weeknights can be tough. But I think if they keep this up, there’s going to be a huge surge this summer.”

Feffer said he expects more than 30,000 fans for Friday night’s game, and that Saturday’s game will likely sell out. Nielsen, the sports marketing expert, said that while the idea of blocking groups from buying tickets “almost flies against everything you’d think was right,” he can understand the Nationals’ perspective.

“From a brand standpoint, and trying to energize the fan base, it’s a good idea,” he said. “But is it going to work?”

That question will begin to be answered Friday night, when Stephen Strasburg takes the mound with Bryce Harper behind him. Whether they are cheered or booed will be the latest indication of what inroads the Nationals — the first-place Nationals — have made in Washington.