The scene was Laurel, the Greene Turtle sports bar, on a mostly deserted Tuesday night. There was a $1.50 special on draft beer, a TV screen as big as a barn door -- and an invisible baseball barrier running between the stools.

"It's right here," said Bernie Berry, pointing out the imaginary line separating him from friends Charlie Gallagher and Jim McElwaine.

Berry supports the Baltimore Orioles; he has for decades. "A fan at 5, a fan at 50," said Berry, 36, meaning he's committed for at least 14 more years.

But Gallagher and McElwaine have gone over to the other side, becoming supporters of the Washington Nationals.

"That's the home team," said McElwaine, 41, of Olney. "What's wrong with rooting for the home team?"

The same line is being drawn across Maryland, especially in counties where Washington's suburbs bleed into Baltimore's. In what used to be hard-core Orioles territory, the appearance of the Nationals has divided families, workplaces and Little League teams.

The volume has been raised in recent days, now that the Nationals and the Orioles are in first place in their divisions.

It's not a blood feud yet -- just a healthy and sometimes cranky rivalry. But underlying the divide is what drives apart fan bases: disputes over history and loyalty, and different conceptions of where home is.

"There's definitely true, hard-line alliances already," said Tim Stevenson, a baseball coach in Severna Park who has seen his adolescent players debating which team is better: the Nats or the O's.

In Cameron Rahnama's family, where he supports Baltimore and his brother-in-law Washington, the teams' winning ways mean both sides have more ammunition.

"We went out to dinner the other night -- he's ranting and raving about how the Nationals are in first place," said Rahnama, the athletic director at Howard's Glenelg High School. "And I'm like, 'The Orioles are in first place, too.' "

It is hard to tell how many people have changed allegiance, because neither team's front office could provide statistics about fans in the border areas.

It's too early to tell whether the Orioles need to make up ground with targeted marketing, Orioles official Spiro Alafassos said.

Anecdotally, however, there is plenty of evidence that Nationals' fans are creating their own identity -- and that the fan base is pushing farther into Orioles territory.

The changes began at Robert F. Kennedy Memorial Stadium, where when the season began, some fans still were following an Orioles tradition and chanting "O! O! O!" during one verse of the national anthem. But boos often followed, and the chant weakened. By this week, the O's were faint to nonexistent.

Heading north from the city, the Nats have had an impact as far away as the Marley Station Mall in Glen Burnie -- near the orange-and-black heart of Orioles country. There, Pro Image sportswear store cannot keep in stock the team caps with the interlocking "DC" logo.

"New Era just isn't producing them fast enough for me," owner Robert Harnsberger said of the cap company.

But the front line seems to have settled in Howard and Anne Arundel counties, where the two big-league ballparks are almost equidistant.

There, the Orioles' monolith has begun to crack along several lines.

One dividing line is age: Some fans of the long-departed Washington Senators said they forgot about all other suitors when they saw a new team with a "W" on their caps.

"All of a sudden, I didn't have any interest in the Orioles at all," said David Paulson, 73, a retired pharmacist from Columbia. "It's the Nationals all the way."

Some fans cited the Angelos Factor, saying that they were sick of Orioles owner Peter Angelos lobbying against baseball coming to Washington.

And in many cases, the allegiance to either the Nats or O's is based on a connection to one metropolis or the other.

"I was always a Washington-oriented person," said McElwaine, who grew up in Largo and has a law office in Greenbelt. "And Baltimore's not Washington."

But then came the usual counterargument: What about loyalty to the Orioles?

"If a football team moved to Loudoun County" near his home in Vienna, Berry told his friend, "I would still be a Redskins fan."

"Different," McElwaine replied. "Different."

For Howard and Anne Arundel fans who do choose the Nats, snippy O's fans are the least of their troubles. More important is the problem with viewing games on TV: A dispute between Angelos and Comcast Corp. means that Nats games usually are not available on cable in those counties.

As a result, fans have found themselves straining to watch tiny televisions and playing with rabbit-ear antennas to bring in the broadcast signal from the District.

In other words, welcome to baseball in the 1950s. And those are the lucky ones. Steve Lalekos , 58, of Arnold can get most Nats games only on the radio.

The radio in his car.

"I can't sit in the car all day," Lalekos said. "I kind of just keep running out and listening to an inning or two."

It has not deterred him. "I'm determined to follow this team, and they're not going to stop me," he said.

For now, the dividing line isn't always clear. Some fans, for instance, are holding on to the belief they can root for both teams.

Such a fence-sitter was Kevin Gibbs, 34, of Rockville, who came to Wednesday's Nationals game wearing a Nats hat and a Brian Roberts Orioles jersey. He said that when the Nationals and Orioles play someday, he will have to root for the O's.

"But I wouldn't be mad" if the Nationals won, he added.

Sports history, however, tends to mock such attempts at neutrality. Try to find a Chicago native who hasn't been forced to choose between the White Sox or the Cubs, or a New Yorker who's neutral between the Yankees and Mets.

Closer to home, consider the fissure between Washington Redskins and Baltimore Ravens fans. In the nine years since the Ravens arrived, their rivalry in the border areas has hardened into a you're-with-me-or-against-me feud.

Barstool prognosticators at the Greene Turtle said the same thing will happen to fans when the O's and Nats start playing each other next season.

"Give it a couple of years," said Jeremy Holbrook, 26, of Crofton. "They'll be bumping heads."

A small experiment in the attraction of Nationals baseball could be made Wednesday night, when Oriole fans Ian Hester and Jackie Fritsch, both 21, came from Crofton to see the Nats play the Oakland Athletics at RFK Stadium.

In the fourth inning, when Nationals catcher Brian Schneider hit a home run, Hester and Fritsch stood and cheered with the crowd.

"It was a nice play, but it doesn't mean anything," Fritsch said. "If you look at the statistics, the Orioles are better."

Orioles fans Ian Hester and Jackie Fritsch, both 21, of Crofton, cheer at a recent Nationals game at RFK.Kevin Elznic, 9, left, and brother Christopher, 14, cheer the Nationals during a recent game at RFK Stadium. "It's the Nationals all the way," says David Paulson, 73, of Columbia, who uses a small TV to get the broadcast signal from the District.