Nationals Park become, in the Phils’ bragging words, “Citizens Bank Park South.” (John McDonnell/The Washington Post)

The relationships between towns and their teams evolve over years, not days, months or even a couple of seasons. Countless episodes are needed to build the steel-strong bond. Philadelphia suffered with its Phillies for more than a century, lost more games than any team in history, before finally realizing in recent years that it was safe at last — in fact, it was a most worthy emotional investment — to love the Phils, not boo them.

That Phillie Fascination, so many generations in the making, arrived here in Washington this weekend in waves that mounted tens of thousands high in the stands at Nationals Park. For three games, the majority of the crowds of 37,841, 44,685 and 41,727 chanted “Lets Go Phillies” and three times they stood in the bottom of the ninth ready to celebrate wins.

Twice, those cheers were jammed back in their throats. Twice a team that the Phils and their fans barely notice — the third-place almost-.500 Nats — ignored the irony that their own Nationals Park had become, in the Phils’ bragging words, “Citizens Bank Park South.”

Twice, including Sunday evening in a 5-4, 10th-inning win on a walk-off-hit-by-pitch by Jonny Gomes, the Nats forged the kind of cardiac comebacks and down-to-the-last-strike wins against a mighty foe that begin to build bonds of their own between a town and its young budding team.

Twice in three packed-house days, on Saturday at 12:25 a.m. and again on Sunday, at the end of two rain-delayed marathons in which the Nationals refused to lose and came from behind repeatedly, those Phillie fans had to close their mouths, fall silent, and ultimately go home losers.

A Washington Nationals fan with the baseball glove grabs a pop foul in a section loaded with Phillies fans. (John McDonnell/The Washington Post)

“You gotta sit back down,” chuckling Nats Manager Davey Johnson said of all those invading fans who had to flop back in their seats en masse when Ian Desmond hit the most difficult kind of comeback homer that’s possible in baseball — down to your last strike, nobody on base. He smoked a liner over the left-field wall (off Antonio Bastardo, 1.48 ERA) to tie it at 4.

“Kind of reminded me a little of ’86 [in the World Series] when the Red Sox were up on the front step [to celebrate] and then they started creeping back.”

It’s tough to decide which of the Nats’ comebacks was more dramatic or improbable. But Washington has gone 2-1, 1-1 and 2-1 in its past three series with the Phils, including a current 5-2 run. In 13 meetings in 2011, they’ve been outscored by a mere four runs by the only team in Phils history with a winning percentage over .640.

Perhaps surprisingly, the Nats don’t expect to be loved by their town yet. Series like this are just building blocks. They’re aiming much higher. But if they succeed, there will be plenty who want to join the ride. For now, Nats fans still seem too gun-shy after 298 losses the previous three seasons to admit — in the presence of Phils fans — that they root for Washington.

During the Nats’ winning rally Sunday, even with the bases loaded and none out, only a few small groups stood and cheered openly. Out of perhaps 25,000 fans left in the park (a 71-minute delay for a thunderstorm thinned the crowd), it seemed maybe a hundred were Nats fans. Yet the instant the winning run was waved home, half the park stood and cheered. Who are they, these secret, ashamed or cerebral Nat fans?

“You have to earn the kind of fans the Phillies have. That takes time,” said Ryan Zimmerman, who capped a six-run Nats ninth inning in the series opener with a two-out, full-count, walk-off grand slam off Phils closer Ryan Madson for an 8-4 victory. (Repeat: Two-out, full-count, walk-off grand slam.)

That was Madson’s second blown save of the year. The Nats battered him for so many pitches he was used up for the weekend. So, Zimmerman began Sunday’s 10th-inning rally against Brad Lidge and stung a double to left, sliding head-first into second — the same kind of slide that put him out for two months.

A Phillies fan calling himself “Cheesesstake Head” is shown. (John McDonnell/The Washington Post)

“Guess that means I’m stupid and starting to feel better,” he said.

Even Jayson Werth, hitting .225 against the Phils this year with a horrid .602 OPS, had his moments. All afternoon, as cheers for one side were met with boos from the other, the only subject of agreement was the value of Werth. He received bipartisan booing every time up. Twice, Roy Halladay struck him out looking with two men on base.

Give Werth this. He’s performed at the level of a “replacement player” — an outfielder you could acquire almost cost-free — for five months. But he grinds. His single on an 11-pitch at-bat started Friday’s rally. His hit to left off Lidge on Sunday loaded the bases in the 10th and set the stage for the win.

When you’re invaded, there are two responses. One is to appease, the other to resist. Even in the microcosm of sports, the comparison holds.

On Saturday and Sunday, the Phils fans jammed Nats Park with the largest and third-largest crowds in the stadium’s history. The Saturday mob was 2,700 above “capacity.” Sunday, the Frederick Douglass Bridge had a 30-minute delay due to the unloading of Phils-fan tour buses. The makeup of this weekend’s crowds prompted Gomes to say to Phillies first baseman Ryan Howard, “You guys should give us your meal money.”

So, one question dominated every game: Would the Nats and their crowds fold when challenged and thus risk mimicking the Orioles’ fate in the last decade?

For years the Birds have been mortified in Camden Yards by huge Yankee and Red Sox throngs annexing their park. The cumulative effect has added to the sense that Baltimore may be a doomed baseball town, subsidized by revenue from the wallets of those who love their rivals.

The past two years, since opening day 2010 when ex-Nats president Stan Kasten begged Phils fans to come on down to D.C., they’ve owned Nationals Park. Before Kasten went to Philly on a media tour to drum up business, the Phils actually drew smaller crowds in Washington than normal Nats attendance (26,219 to 27,249 from ’05 through ’09). Ever since then, Phils crowds here have been 25 percent above the Nats’ norm and far louder.

But never like this weekend.

Then they got sent on the long trip back home in walk-off silence — twice.

“The good news is that it’s temporary,” said Kasten, interviewed by phone during the game. “The Nationals are about to get a lot better. And the Phillies aren’t going to stay this good forever. Both things are in the process of changing. As they do, the crowds at those games will change.

For now, the Nats themselves have clearly chosen resistance — and at a spectacularly dramatic level — as the proper response to invasion.

The Nationals fans, not so much. But things change. That may, too.