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A Mets fan’s sign echoed a slogan from 1973, when the team won a pennant.Credit David Zalubowski/Associated Press

Noticing more blue-and-orange caps and fewer navy pinstripes around New York these days? Hearing more talk about how the Mets keep finding ways to win?

It could be that the Yankees’ seemingly unshakable hold on the city’s baseball heart is loosening amid the sudden and stunning turnaround for the Mets.

Both teams may be headed for the postseason, so another test of popularity may be coming soon. And measuring the pulse of a fan base in a two-team baseball city is never simple, especially when one of them is the Yankees, with their 27 World Series championship and 20 retired numbers.

But some telling evidence points to trouble for the Yankees and a boon for the Mets, suggesting that New York might be turning into a Mets town for the first time since their championship season of 1986.

“It certainly feels like something’s happening,” said Greg Prince, a blogger for the website Faith and Fear in Flushing. “Winning certainly can change the equation, especially winning that hasn’t happened in a long time. It’s not that the other team in New York isn’t winning, but there doesn’t seem to be a lot of spark to it, while what’s happening to the Mets feels fresh and novel.”

It is not just a feeling. By the measures of attendance and television viewership, the Mets are surging while their crosstown rivals are sliding a bit. It is an improbable reversal of fortune, given that the Yankees have dominated the market so clearly since they won four World Series from 1996 to 2000, capped by a triumph over the Mets in the so-called Subway Series.

The Yankees’ paid attendance at home is averaging 39,537 a game, down 5.6 percent from the average at this time last year, according to Baseball-Reference.com. The Yankees, who trail first-place Toronto by three games in the American League East, have never averaged below 40,000 fans a game since moving to the new Yankee Stadium in 2009.

The Mets are averaging 31,257 a game this season, a 17.6 percent rise from last season. That is still about 10,000 short of the capacity at Citi Field, but this season’s increase of 4,689 fans a game represents a drastic shift from a dispiriting trend: Attendance had fallen almost 32 percent from a peak of 38,941 during the inaugural season of the ballpark six years ago.

Yet perhaps a more precise reflection of the passion of a fan base is viewership on a team’s cable television channel. After all, most fans prefer to watch games without having to buy tickets, which can be expensive.

The Yankees’ YES Network started in 2002, soon after the peak years of the dynasty. The Mets’ SNY began in 2006 and then capitalized on a three-year period when the Mets were a good team, albeit one that endured heart-wrenching late-season collapses in 2007 and 2008.

For most of the past six years, there was no doubt that the Yankees were a better television product than the Mets, who spiraled into a skein of losing seasons.

But the Yankees, who averaged 454,000 viewers a game in 2007, are drawing only 256,000 this season, a 10 percent decrease from 2014 after a comparable number of games.

The Mets’ average television audience, which reached a high of 314,171 in 2007, bottomed out at 138,627 in 2013 before a slight revival to just over 144,000 last season.

But so far this season, viewership is up 62 percent, to 240,091 a game. And games are averaging 324,195 viewers since the Mets acquired the slugging outfielder Yoenis Cespedes on July 31.

For the season, the Yankees’ lead over the Mets in average viewership is about 20,000 — a far cry from four years ago, when the difference was more than 200,000.

Much of the surge in attendance and viewership for the Mets can be easily explained. The Mets are suddenly a fascinating, charismatic, winning team, built on the powerful young pitching that the team long promoted as its eventual main attraction, and an offensive attack that is now as powerful as it was anemic before late July.

“This is a team that just doesn’t know how to lose,” Gary Cohen, the SNY play-by-play announcer, said during the Mets’ come-from-behind victory over Atlanta on Sunday.

Jim Breuer, a comedian and Mets fan who posts passionate videos about the team on Facebook, was watching the game, gleefully responding to how the team tied and then went ahead of the Braves.

“I was like, ‘All right, we have to lose once in a while,’ ” he said by phone while on his way to Monday night’s game against Miami.

Shifting Fortunes

The Yankees’ lead over the Mets in average television viewership has narrowed to barely 20,000, a far cry from four years ago, when the difference was 10 times as much.

Shifting Fortunes

The Yankees’ lead over the Mets in average television viewership has narrowed to barely 20,000, a far cry from four years ago, when the difference was 10 times as much.

“Then Lagares hit the double and Granderson walked, and you just knew it; you just knew it,” he added, referring to Juan Lagares and Curtis Granderson, who started the Mets’ ninth-inning rally.

Breuer said that when Daniel Murphy came to the plate, “I said, ‘He’s going to belt a homer,’ and when he did, I laughed — I just couldn’t stop giggling.”

The Yankees are an older, less flashy team that lost much of its charisma with the retirements of Mariano Rivera and Derek Jeter in 2013 and 2014. The former ace C. C. Sabathia is struggling with a bad knee and the wear and tear of pitching nearly 3,000 innings in his career, and the Yankees’ current top pitcher, Masahiro Tanaka, is soldiering on with a slightly torn elbow ligament.

Their biggest star is probably Alex Rodriguez, whose unlikely comeback after a season-long suspension has helped keep the Yankees in the pennant race. Although it seems that fans have grudgingly accepted him because he is producing well and not causing trouble, he is not a Jeter-like presence who draws fans to the stadium in droves.

Jeter merchandise is the hottest seller among all Yankees, according to Fanatics.com, a retailer. The Yankees are the top-selling team in Major League Baseball this season, but sales of Mets goods, led by those of pitcher Matt Harvey, are up 140 percent this season and 300 percent this month compared with figures from 2014.

Still, the task of turning New York into a Mets town is far from complete. Prince, the blogger, offered two ways to measure future progress: when Mets caps are highly visible on all trains, not just the 7 line to Flushing, and when a fan’s request to turn a restaurant television to a Mets game is not met with a look “like you have three eyes.”

More important, they probably need to do well in the postseason and then re-sign Cespedes.

“I think we have a great chance of going all the way,” Breuer said.

Before that can happen, the two teams will play each other this weekend for the first time since April, when the Yankees took two of three games.

They may want to remind the Mets again of who has been boss for so many years.