HE GOES it alone, no matter how many prized arms they import, no matter how often outsiders ruminate about the burden being lifted, as if some Baseball Sage this summer approached Al Leiter and removed the “Ace” mantle from his head.

Tomorrow afternoon, when he stands atop the brand new mound at Shea Stadium, it is Leiter’s job to make the Mets’ home– and American — opener a success when he faces the Padres. He promises his mind will not drift back to the last time he pitched in a game of consequence, when he did not retire a single batter on that ultimately fateful Oct. 19 night in Atlanta.

He insists the knowledge that Mike Hampton is now on board will not trigger some sort of emotional unburdening. And, upon reflection, he admits the start of this season, his 11th in the big leagues, has already generated more excitement inside him than any other.

He knows what it feels like to embrace the springtime believing he’s with a club that can play deep into Autumn, but this time, with these Mets, something more precious might be at work.

“To compound it a little more and make it a little more special is that I grew up a Mets fan, started my career in New York, there’s a good chance I’ll hopefully end my career here with the team I rooted for as a kid,” Leiter said. “So yeah, this is more exciting.”

There’s something boyish about Leiter, who at 34 is certainly not the 22-year-old kid who first appeared on the New York scene as a Yankee flame-thrower. A certain youthful quality is a constant around a pitcher who is every bit as important a player for the Mets than anyone else in uniform, Hampton included. In some ways, the rise and fall of the Mets hinges more delicately on Leiter, who has this confounding habit of pitching excellence every-other year.

This would be the on year for Leiter, as he was 17-6 with an earned run average of 2.47 in 1997, his first season with the Mets. The dip was dramatic last season, down to 13-12 and 4.23. A popular theory circulating is that with Hampton on hand as the unquestioned ace of the staff, Leiter can slip more comfortably into a support role, no longer asked to be the starter who sets the bar for others to reach.

It’s a nice, sensible theory but it’s also more fiction than reality. No doubt, pitchers can feed off each other’s success. And, from time to time, Leiter will be relieved of the challenge of facing the opponent’s top pitcher, a responsibility that Hampton now adopts. But the mound is a lonely place, and when Leiter is on it, he’s on his own island, and Hampton cannot be there to keep him company.

“It really doesn’t directly affect me at all,” Leiter said of Hampton. “I know so many people are talking about how much he’s going to help me. Where Mike helps me is he’s a lefty in the starting rotation, knowing I can talk to him about certain hitters. Mike sitting on the bench is not going to help me with my slider, it’s not going to help me get good arm speed on my changeup.

“To me, I feel every bit as nervous, every bit as excited, every bit the anticipation and expectation of wanting to do the best I can. I want to have a great year. Whether Mike was here or not, those would be the feelings I have.”

Not that Leiter is anything but thrilled that a talent like Hampton has arrived.

“Anyone who can help me get a fourth championship ring,” Leiter said. “That’s all I think about. I’m pretty selfish. I won two in Toronto and one in Florida, and I want one with a plain NY on the ring, instead of a bird or a fish.”

The whole notion of who’s the ace and who isn’t is not embraced by Bobby Valentine, who says all pitchers operate at times in a vacuum.

“I don’t understand it,” Valentine said. “The guy’s got to pitch his game. Al’s going to pitch against a lot of guys who are considered the No. 1 starter of the opposition, and so will a lot of my other guys, and I’d hate for them to think they’re at a disadvantage at any time.”

The company line, and a correct line, is the one issued yesterday by Mike Piazza: “When you have somebody like Mike Hampton, there’s no question he’s going to make everybody on the staff better.”

In a team sense, that’s true, but Leiter will not think of himself as one of the supporting cast, and he can’t perform like that if the Mets are to fulfill their lofty expectations. While his teammates were traveling across the world to open the season in Japan, Leiter remained in Florida, resting his body and mind for his home-opening start. He managed to wake up in time to catch the third inning of both games.

“I have an alarm clock and her name is Carly and she’s three-years old,” Leiter said.

The wake-up call comes at a more resonable hour tomorrow. Leiter doesn’t need to be called the ace. He just needs to pitch like one.