Eight other horses will be ready, too, looking to thwart Smarty's bid to become the first horse to win the Triple Crown since Affirmed in 1978.
Of the 28 horses that have entered the Belmont with a chance at the Triple, 17 have failed, including five in the last seven years.
Charismatic broke his leg during the race in 1999, War Emblem stumbled out of the gate in 2002, and Funny Cide floundered in the mud last year. And in 1979, heavily favored Spectacular Bid lost after stepping on a safety pin the morning of the Belmont.
Jason Orman, who saddles Rock Hard Ten, said the rain adds an element of surprise to every race.
"You never know how they'll handle it," he said.
The rain alone shouldn't be enough to keep Smarty Jones from making history before an expected crowd of more than 100,000. He does well on wet tracks, winning the Arkansas Derby on mud before doing the same at a drenched Churchill Downs.
Forecasters predicted a 60 percent chance of rain at the 6:38 p.m. post time.
"I know that my horse is a good horse, and he'll run on anything," Servis said.
"The country has been waiting for a Triple Crown winner, and I know now why there haven't been many. The American people have fallen in love with this horse."
Seven months ago, no one outside Philadelphia Park had ever heard of Smarty Jones. Now he's the 2-5 favorite in the race that could make him just the 12th Triple Crown winner.
Since slamming his head on an iron bar at a starting gate last summer, fracturing his skull and almost dying, Smarty has been overpowering in winning all eight of his races under the patient and poised riding of Stewart Elliott.
And the undersized red chestnut colt seems to be getting stronger. Three weeks ago, the Pennsylvania-bred horse owned by Pat and Roy Chapman won the Preakness by a record 11 1/2 lengths, just two weeks after beating 17 rivals in the Kentucky Derby.