The 6-year-old gelding appeared to tire in the stretch but after roaring through a mile in 1:35.71, he had a right.
"How can you explain a race horse this big that feels 10 feet tall?" wondered jockey Todd Glasser after the race. "You got me."
Presidentialaffair, who once ran for a $20,000 claiming tag, went head-to-head last year with horse of the year Ghostzapper, and trainer Martin Ciresa said he has come back even better this year.
Despite the presence of fast runners such as Classic Endeavor, Lusty Latin and Hydrogen, Presidentialaffair never was challenged in the McKay.
"This is the best race ever," Ciresa said. "But he's a lot of work. I don't want people to think it's easy."
Also Saturday, 4-year-old filly Silmaril came from off the pace to win the $100,000 Northview Stallion Station Stakes, running the 1 1/8-mile race in 1:53.71.
The Maryland-bred daughter of sire Diamond has now won eight of 13 lifetime starts for trainer Christopher Grove.
LEXINGTON STAKES: Coin Silver pulled away in the stretch in Lexington, Ky., giving trainer Todd Pletcher a third horse to run in the Kentucky Derby. Last week, Pletcher's Bandini won the Blue Grass Stakes and Flower Alley finished second in the Arkansas Derby. Coin Silver was ridden by Javier Castellano, who also rides early Derby favorite Bellamy Road, so Pletcher will likely have to find a new rider. Rockport Harbor, trained by John Servis, who won last year's Derby with Smarty Jones, faded in the stretch and finished sixth. The loss likely will keep the 3-year-old colt out of the Derby in two weeks.
The Associated Press contributed to this report.