For the foreseeable future, Bryce Harper has tied together two fan bases that already disliked one another. (Alex Brandon/Associated Press)

Sitting on his couch April 2, Kevin Marcou felt his head give way to his heart, which was thumping “crazy hard” as he watched Bryce Harper stride to the plate at Nationals Park for the first time as a visiting player. Marcou didn’t begrudge Harper signing in Philadelphia, even though that city harbors the fans his fellow Washington Nationals supporters dislike the most. He thought Harper earned his contract, that things in Washington just hadn’t worked out. But now, seeing Harper in that Phillies uniform, something popped.

“All these years, I’ve been a dispassionate, rational-thinking guy, but …” the 33-year-old from Montgomery County, Md., paused as he considered Harper’s first at-bat. “I don’t even know how to describe it. It felt like a wave of passion came over me in that moment. I’d never felt anything like it. Not in [Jordan Zimmermann’s 2014] no-hitter. Not in Max [Scherzer’s two 2015] no-hitter[s]. I’m usually just a passive observer, but in that moment, I just felt angry. It was weird.”

Harper has made many Nationals followers the emotional fans they claim not to be and emboldened Phillies supporters, who waved “We Got Him!” signs at that April 2 game and harked the revival of the Nationals’ ballpark as “Citizens Bank Park South.” This year, though, Washington and Philadelphia could become meaningful on-field rivals for the first time.

The two clubs have experienced opposite fortunes over the Nationals’ entire 15-season existence, only twice finishing closer than 13 games apart in the National League East and never playing a September series with significant stakes for both sides. But after just five games against one another this season — in which the Nationals are 3-2 after administering a 15-1 shellacking at Citizens Bank Park on Wednesday — that seems destined to change.

The Nationals have demurred when asked about a rivalry — “I don’t worry about the Phillies … I worry about those guys right there in our clubhouse,” Manager Dave Martinez said — but they have played the most dramatic April baseball some Nationals players and Martinez could remember. Closer Sean Doolittle noted each win feels “a little more important than in years past.” Some fans in Washington are already steeling themselves for the Nationals’ second-to-last series of the season, four games against the Phillies at Nationals Park on Sept. 23-26. Then there’s Harper, the man who will likely tie these franchises together for at least the next 13 seasons.

“It’s a whole new rivalry,” said Jenny Harper, a nonprofit vice president not related to the MLB superstar, who has rooted for the Nationals since their return to the District. “It’s about to get uglier than it has been in a long time.”

She attributed the budding rivalry to three causes: Unprecedented competitiveness between the Nationals and Phillies, the fresh wound of Bryce Harper’s departure and the proximity of two culturally different cities. Jenny Harper also predicted increasing animosity between the fan bases because she remembered what Nationals Park was like during the Phillies’ golden era, when it seemed like all of Philly bused down to “terrorize” Nationals fans.

From about 2009 to 2011, she stopped attending Phillies games at Nationals Park because it became too much. Other fans shared her grievances: Phillies fans were “inappropriate,” “intimidating” and “obnoxiously drunk,” they said. Once, she remembered, she was in a section with Phillies fans who spent the game screaming obscenities at hometown fans including her and her husband for cheering their team’s players.

“Those of us who have been around, earlier this decade, it sucks” to have more Phillies fans coming to the ballpark, Harper said. “Phillies fans bring out the worst in Nats fans.”

It got to a point where, in 2012, the Nationals sent a letter to fans saying, “Let’s work together to keep Phillies fans out — it’s time to TAKE BACK THE PARK!”

For their part, many Phillies followers see themselves as maligned by a sensitive fan base. A few Phillies fans, such as Jennifer Thew, believe it’s been a “good rivalry” since outfielder Jayson Werth signed with the Nationals in December 2010 after spending four seasons with the Phillies. Others, including Marvin Lockett, believe Werth wasn’t a prominent enough figure and denied a rivalry was developing down Interstate 95. Another, Patrick Kelleher, said the Nationals organization is too young, and said he is still more worried about the Atlanta Braves and New York Mets.

“They’ve never even played in the NLCS,” Kelleher said of the Nationals. “Once they beat us at the end of the year, then maybe. But it’s going to take a couple years [to develop].”

Some Washington fans countered that, for all of the blustering about blue-collar loyalty to Phillies fandom, “Citizens Bank Park South” disappeared as soon as the team started losing more than 90 games per year. And in the eight years since Phillies fans have come to Washington en masse, Jenny Harper said, Nationals fans have changed. They’re growing up. They’re more engaged. The boos toward Bryce Harper told her that. She wondered if Nationals fans, who she always found quiet and reserved, might be ready to get in some faces.

Marcou feels the same way. He understands the hostility toward Bryce Harper will fade, but he said for as great as he felt when Scherzer struck out Harper, he hurt as strongly when Harper deposited a tape-measure home run into the right field’s second deck — a feeling as bad as any he experienced over the many playoff disappointments throughout Harper’s time in Washington. If this was how he felt in one game, he didn’t know what to expect for the rest of this season, let alone beyond.

“We’ve never felt like this before,” he said. “I’m curious to see what it’s going to do to our fan base and this rivalry. It’s a real thing now.”

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