The Sunday night game will be nationally televised, and Mr. Feffer acknowledges it would be a huge embarrassment if the crowd were dominated by Philadelphia supporters. He recently sweetened the pot, offering two free tickets to future games against other teams to anyone who buys two tickets to the Phillies series — a move designed to appeal to locals. Nationals officials said Wednesday the moves appear to be working and that the club’s internal numbers show fewer out-of-area ticket sales than in years past.

Should Phillies fans show up in large numbers, however, Mr. Feffer says he has other tricks in mind to drown them out. Without elaborating, he says, “We are not done yet.”

More importantly, D.C. Mayor Vincent C. Gray has declared the upcoming weekend “Natitude Weekend,” according to a Tweet sent out by the club.

Among other things, the proclamation says that this weekend, “all local residents show support of their hometown team,” and that “Nationals players represent a ‘Natitude’ worthy of emulation,” and that “every day the Nationals inspire our residents to ‘Ignite Your Natitude.’ ”

So now it’s official.