The Mets have stood at this portal before.

You may remember how they were going to be carried back into the warm embrace of this city by Generation K. It was an idea that so frightened George Steinbrenner he demanded after the 1995 season the Yankees go with a kid shortstop in 1996 just to show they had a farm system, too.

Derek Jeter will be at Target Field on Tuesday night for his final All-Star Game. We will get back to you as to the whereabouts of Generation K.

Then there was a time — not long ago it seems — Jose Reyes and David Wright were going to usurp Jeter and Alex Rodriguez in ability and five-borough affirmation. And — twisted history of A-Rod or not — that didn’t happen, either.

The spending spree that brought Carlos Beltran, Carlos Delgado, Tom Glavine, Pedro Martinez and Billy Wagner fueled an NL-best record in 2006 and dreams the Mets — finally — were in the left lane, passing the Yankees. Alas, that had about the same shelf life as Tim Tebow, NFL superstar.

Then there was the time when the Mets — not the Yankees — won the trade bidding for Johan Santana. The good old days were here again. But, of course, they weren’t.

In other words, we have been here before. At that moment when we wonder whether the New York teams were Subway trains passing in the night, racing in different directions. After two-plus decades of almost complete pinstripe dominance, there is a natural inclination to just believe the Yankees always will find a way — or a pay — to extend their dominance in this region, to remain princes of this city (maybe even the Kings of Queens). And that the Mets will continue their patent on baseball humiliation.

In this town, the Yanks have been made men, the Mets Madoff.

Yet, cue familiar music, we are at another moment when the Mets and their disheartened fan base can dream a little — of better days finally being near, if not here. Meanwhile, the Yankees are trending toward the dreaded Double Bs — bad and boring. There is nothing a fan in this city loves more than his team being up — unless if it can coincide with the other New York club being down. This has been the Yankee Way since the mid-1990s, since Buck Showalter and Stick Michael transformed the culture and talent from dread to dream, since the Core Four arose nearly en masse, the most talented quartet the Big Apple had seen since The Beatles played Shea in ’65.

The Yankees, indeed, became a rock band; a traveling superstar show that had to be seen, replete with regular gigs in the Canyon of Heroes. A self-perpetuating machine that addicted itself to October baseball during a period when the Mets often were done by August.

Let’s be fair where we are at exactly this moment: The Yanks are a .500 team, five games out of first place and with a history of going for it. The Mets are five games under .500, seven games out of first and with a hierarchy that hasn’t exactly endeared itself in these parts.

Again, if history is any lesson, Shane Greene is going to be Aaron Small and Chris Young is going to squirt bleach in the clubhouse.

But it is hard to ignore the trend lines, and not just that the Mets won four-fifths of a homestand (eight out of 10) to close the first half while Masahiro Tanaka’s elbow injury meant the Yankees were now without four-fifths of their season-opening rotation. It is deeper than that.

It is that the Yanks lost Tanaka — at least for six weeks, and maybe until 2016 — when he had become the reason, at least once every five days, to watch the Yankees and believe they were more than an expensive bust. It is that Jeter’s career is coming to an end, the last tie to a beloved era in Yankees history. He is a half season away from being nostalgia, like Gehrig or DiMaggio.

The Yanks have tried mightily and financially to extend the good times, but with ebbing success. No one has stepped into the forefront as the Next Great Yankee to take the Jeter baton. And, in many ways, the perhaps unrepeatable two decades worth of success — combined with a new stadium that in pricing and sound-draining architecture has helped rob the franchise of aura and mystique — has ushered in Yankees fatigue. Nothing they do or spend is enough right now.

They are just 18-23 at home and open the second half with 10 games in The Bronx that should define — at least for 2014 — their path. But their docile farm system has forged larger and longer-lasting issues for the franchise. Like a bad political policy repeated, the Yankees keep trying to spend their way out of a problem that cannot be solved simply with a fat wallet, or else you end up watching too many CC Sabathias and A-Rods crumble or Beltrans and Brian McCanns fail.

The Mets, of course, are not into the sunlight. There are other clubs — the Cubs and Astros come readily to mind — who believe they have set themselves up with a deep farm system and a made near future. It won’t all work, and Mets history is always worrisome on this kind of subject (again, see Generation K, among others).

But the pitching is deep and promising in the majors, minors and with Matt Harvey readying to return in 2015. The hitting, too, is getting better at multiple levels. The Yanks spent a couple hundred million dollars to infuse their offense last offseason, yet are averaging the same 4.0 runs per game as the Mets.

Suddenly, Travis d’Arnaud and Lucas Duda look like part of a solution and the Mets actually are getting out of Yankees refugee Curtis Granderson what the Yankees are not getting out of Mets alum Beltran. When in the name of David Cone, Dwight Gooden and Darryl Strawberry has that ever happened? The third-place hitter for the AL All-Stars Tuesday night has been the Yanks’ best hitter in recent vintage. But Robinson Cano is now a Mariner. The New York second baseman in the All-Star Game is Daniel Murphy.

We will see if the Mets translate a wave of good vibes and strong possibilities into a positive era. We will see if the Yankees can once more find a way to outperform their problems and stay contenders, as always.

But at the break point of the 2014 season, it is not far-fetched to believe New York can be a National League city again. The Mets are at the portal once more. What will they do this time?