Annmarie Edwards watched transfixed in her Rockville, Md., living room last Tuesday as the Baltimore Orioles defeated Toronto to secure the American League East title.

Not far away, her older brother Martin Sarsfield was equally riveted to a different TV channel, as the Washington Nationals beat Atlanta to win the National League East.

The two — loving siblings who grew up as baseball fans together in Silver Spring, Md. — are a hardball house divided, and they are hardly the only ones.

Ten seasons after the Nationals arrived in what used to be the Orioles’ southern annex, the parallel surge of both teams has raised more than the prospect of a Baltimore-Washington World Series (dubbed “MARC Madness” by one clever reader). It has exposed an Interstate-95-wide rift between those who have adopted the curly W and those who have stuck with the O.

“It’s kind of appalling to me,” Edwards, a 49-year-old public relations consultant, said of her brother’s seamless switch to the new home team in 2005. She remembers him as an Orioles fan so avid that he knew the names of ushers at Memorial Stadium. “To me, it’s the total abandonment of his roots.”

Mapping Nationals-Orioles fandom

But to Sarsfield, a 52-year-old risk manager, there was no question he would give his heart to the D.C. team he had been yearning for since the Senators franchise left town in 1971, when he was 9 years old.

“I never really thought we’d get another team,” said Sarsfield, who now lives in Northern Virginia. “I resigned myself. I loved the Orioles. I love Baltimore. But I’m not a Baltimorean, I’m a Washingtonian.”

Bringing a new franchise to the District was supposed to give fans an option. But for many, picking a team has been more a matter of conscience than choice, leading to divisions in living rooms, offices and sports bars around the region.

To some, fandom is a matter of civic identity. To others, it’s mating for life.

“My husband asked me why I didn’t just switch,” Edwards said. “I said, ‘I’ve been married to you for 20 years and if some new guy came along, I wouldn’t just leave you and go off with him.’ ”

Sam Howard came of baseball age when the O’s were the only team in town, even though it was another town. The 21-year-old from Takoma Park, Md., now a senior at La Salle University, never considered switching allegiance.

“Camden Yards was my first park,” he said. “When the Nats came to town it was exciting, but it was like, ‘We’ve already got a team.’ ”

Nationals players celebrate their second National League East title in three years after a 3-0 victory over the Atlanta Braves on Tuesday. (The Washington Post)

His father, on the other hand, didn’t think twice. John Howard, a jazz musician and music teacher at the Edmund Burke School in the District, went to buy his first Nats cap when the team store was a trailer in the parking lot at RFK Stadium. After growing up a New York Yankees fan in New Haven, Conn., and then following a team located 40 miles up the interstate, he was ready to root for a team where he lived for a change.

It was love at first pitch.

“It really felt like they were my hometown team,” he said. “People are wearing Nats gear around town, at work. There’s a buzz to it.”

At first, John lobbied his son to cross over with him, but now they are comfortably settled in different camps. They go to games at both parks and listen patiently while the other babbles on about his team. Mostly patiently.

“He always wants to talk about the Nats,” Sam said. “It does get a little annoying sometimes.”

Nicole Schechter, a Baltimore-born psychology fellow, said she’s still negotiating the schism with her O’s-fan-turned-Nats-fan boyfriend, Howie Goldstein, a marine biologist. Their Silver Spring bedroom features competing O’s and Nats decor and, as the season has heated up, they’ve grown a little clingy with the remote control.

“He came in when I was watching the postgame Tuesday night and said, ‘Are you really going to make me watch them hug each other for 25 minutes?’ And I said, ‘Yes, I am.’ ”

Orioles owner Peter Angelos, who once operated an Orioles ticket shop in Farragut Square in downtown Washington, fiercely resisted the relocation of the former Montreal Expos to the District, famously declaring: “There are no real baseball fans in D.C.” And during the Nats’ early seasons, plenty of traces of Birdland did linger, with Orioles bumper stickers common and Baltimore’s signature cry of “Oooo” clearly sounding during the national anthem at sparsely attended Nats games.

But in recent years, especially since the Nationals reached the playoffs in 2012, attendance has soared and the region has become saturated with red-and-white “W” stuff.

Paid attendance at Nationals Park will top 2.5 million this year, putting the team in the top 10 in Major League Baseball, according to the team. The average crowd has risen from 22,000 per game to 31,000 since 2009. Season ticket sales have doubled in the past three years, as have merchandise sales, although the team would not release specific numbers.

“It’s not a novelty anymore, it’s not a new stadium,” said Valerie J. Camillo, the Nationals’ chief revenue and marketing officer. “This is a solid fan base built on solid season ticket plans. We’ve drawn Washington in.”

Data provided by Facebook on the distribution of “likes” for each team show that the region once dominated by the Orioles is now deeply split. The Nationals are supreme inside the Beltway and south of the Potomac; the Orioles rule north of the Howard County line. The teams are vying for supremacy in Montgomery County.

Nanny O’Brien’s, an Irish pub in Northwest Washington, has long been known as an Orioles hotbed. Natty Boh is served in cans, and a portrait of the former Orioles manager Earl Weaver hangs over the bar. But in recent years, the Cleveland Park watering hole has had to adopt both teams. Customers complain if Nats games aren’t on at least some of the TVs that used to be locked on Baltimore broadcasts.

“Our employees are split now,” said manager Mike Johnstone, a lifelong O’s lover. “Half of us are O’s fans, half are Nationals. We have one Phillies fan, but he doesn’t count.”

For the playoffs, the bar will fly both Nats and Orioles flags over the door, and it will replace its usual six draft beers with three from Washington brewers and three from Charm City.

“We’re known as an Orioles bar, and we’re going to embrace that,” Johnstone said. “But the Nats thing has really grown. You have to cater to the local fans, too.”

Teachers say the first generation of Nats babies are firmly on board. Nick Monaco, 27, is the baseball coach at Deal Middle School in the District. His players, some of whom were 4 when the team arrived, are all Washington fans, even as some of their parents still look to the north.

“The older folks in the stands still have the black-and-orange hats and the bird shirts,” Monaco said. “The kids are overwhelmingly with the Nats.”

O’s fans say they didn’t feel like outliers at first. Edwards said she was still surrounded by fellow fans after the Nats arrived, although she resented that local talk radio and The Washington Post sports section suddenly “pretended we didn’t exist.”

But in recent years, as the Nats fans spread, Orioles Nation seems to have pulled its border northward. Edwards’s O’s postings on her Facebook page get only a few likes. Hers is the only bird banner on the block.

“You can’t walk into a Target in Montgomery County and find any Orioles gear,” she said. “It’s all Nationals stuff. You have to go all the way over to, like, Columbia.”

Her brother, meanwhile, said the split has not ruined any Thanksgiving dinners, and that both siblings wish the other team well (unless they should meet in the World Series).

His only complaint is the resistance he’s felt from his nephews at the Ryan Zimmerman jerseys and “W” caps he has bought them over the years. His sister has “brainwashed” them, Sarsfield said.

“You want to tell them, ‘You know, you don’t have to do everything your mommy says.’ ”