THE 139th Belmont Stakes tomorrow, by near unanimous consent among horsemen, boils down to a simple proposition: Curlin vs. The Rest.

To a man, they believe the giant chestnut, fresh from his squeaker win in the Preakness, is the one they all have to beat. They are not worried about each other, they just fear the Arkansas Tornado.

But then everyone thought the same thing three years ago, when they pounded Smarty Jones down to 1-3 in the betting, only to see him up-ended by the 36-1 bomb Birdstone.

They thought Funny Cide was an even-money certainty in 2003, only to see Empire Maker take him to the cleaners. They slammed War Emblem to a 6-5 sure thing in 2002, only to see him stunned by the 70-1 carpetbagger Sarava.

All of which proves that the Belmont, at a mile and a half, is a minefield. Every horse is trying it for the first – and usually the last – time. No one can be sure of anything. To take 6-5 or less about any horse, Curlin included, is a hard sell.

So I stopped by trainer John Parisella, an astute judge, to get the dope on this Belmont. Two years ago he told Post readers that if Afleet Alex got a competent ride from jockey Jeremy Rose, he’d win “by 10 lengths.” Alex got the ride – and won by seven lengths.

This year, he thinks we could see a repeat. “Remember Curlin stumbled at the break in the Preakness, but still won,” Parisella said. “If he gets a clean break in the Belmont, he will sit behind the speed, get to the stretch – and explode.”

The morning line odds, however, suggest a much closer contest. They have Curlin at 6-5 with Hard Spun at 5-2 and Rags to Riches at 3-1. That’s not the line of a one-horse race.

The case against Curlin is speculative: The Belmont will be his fifth race in 12 weeks. The Preakness was a gut-wrenching race and his fastest. Will he bounce?

He cleaned the clocks of weak competition at Oaklawn, but since shipping East he has finished a distant third in the Kentucky Derby and was life-and-death to beat Street Sense in the Preakness.

Now he’s coming into New York and as the late Woody Stephens used to say, “When you cross the river, those buildings get awful big.”

Curlin’s biggest negative of all will be the short price.

Hard Spun is the sentimental choice here but his ability to get the mile and a half is suspect. In his last four races, he has lost ground in the stretch in three of them – at a mile, a mile and a quarter and a mile and three-sixteenths. What price will he pay at a mile and a half?

The biggest threat to Curlin might be Rags to Riches, easily the best filly in the land, but now tackling males for the first time. She has a powerful closing kick, her best Beyer speed fig is as good as Hard Spun’s, she gets a five-pound weight break on the males (a huge asset over a mile and a half) – and top jock Garrett Gomez was anxious to get off Hard Spun to ride her.

Win or lose, her participation in the Belmont is the stuff of dreams.

Three horses – C P West, Imawildandcrazyguy and Slew’s Tizzy – between them have not won a single Grade 1 race, so they appear outclassed.

The best long shot might be Tiago, who has been shipped from California by a trainer, John Shirreffs, who does not take potshots in the dark.

He brought Giacomo east to win the Kentucky Derby at 50-1 a couple of years ago. He brought Tiago to the Derby this year after winning the Santa Anita Derby and saw him close with a furious rush after being checked and losing his momentum at the half-mile pole. The chart shows he lost nearly five lengths at that point.

Since then, the horse has trained nicely. Shirreffs said yesterday. “We have a nice horse, he has not run his best race yet, he has a mile and a half pedigree and with his best race we have a really good chance to win.”

Larry Jones, who trains Hard Spun, said he is wary of Tiago. “John would not have brought him here if he thought he did not belong,” Jones said. “He’s going to be a factor. I’ve seen John here several times and I hate it because he looks so confident. He’s very relaxed and that worries me.”

But the bottom line is that Curlin is the one who worries them all.