Clinton did not suffer a heart attack, but tests earlier in the day at a suburban hospital found four blockages in his coronary arteries that could trigger an attack if left untreated.
"He's in excellent hands, and he's at one of the great hospitals in the world," she said earlier as she cut short a visit to the New York State Fair in Syracuse to join her husband.
The nation's 42nd president, who served from 1993 to 2001, battled his weight for many years and had a well-known fondness for fast food. But he has been on the South Beach low-carbohydrate diet for at least a year and exercises with a personal trainer.
Coronary artery disease claims almost a half-million lives a year in the United States; it is the leading cause of death.
During his presidency, medical tests indicated that his total cholesterol and blood pressure were normal, though his so-called "bad cholesterol" was slightly elevated. Experts noted yesterday that many people develop heart disease whether or not they eat healthy foods, exercise or have risk factors that show up in routine testing.
President Bush extended Clinton "best wishes for a swift and speedy recovery."
Campaigning in Newark, Ohio, Democratic presidential nominee John Kerry told a crowd outside the town hall he was confident Clinton will pull through after surgery.
"I want you all to let a cheer out and a clap that he can hear all the way to New York," he said. "He's going to be fine."
Clinton had made a rousing speech for Kerry at the party's national convention in July and had planned to campaign for him. He also has been on a book-signing tour for his memoir, My Life, and had been scheduled to appear next week in White Marsh.
Experts said Clinton will likely have to avoid strenuous activity for six to eight weeks but could probably make campaign appearances in a few weeks without jeopardizing his health.
Clinton, who had apparently never before experienced symptoms of heart diseases felt discomfort in his chest Thursday, but tests that day at Westchester Medical Center outside New York found nothing wrong.
"We talked through the day and he said he felt fine and not to worry," Hillary Clinton said.
He spent the night at his home in Chappaqua, N.Y., but further testing early yesterday at the suburban hospital prompted doctors to recommend heart surgery. In the test, called an angiogram, doctors inject a dye into the bloodstream and then take a picture of the heart to reveal any blockages.
The test revealed "multivessel coronary artery disease, normal heart function and no heart attack," Dr. Anthony Pucillo, who performed the procedure, told the Associated Press. Pucillo said the blockages were serious enough to warrant an operation.
But heart specialists who were following news of Clinton's illness said the fact that doctors did not rush him into surgery indicated that his life was not immediately in danger.
"That they are postponing the surgery means that he's doing relatively well," said Dr. Luca Vricella, a cardiac surgeon at the Johns Hopkins Hospital in Baltimore.
Vricella said he could infer that the president did not have continuing chest pain or a heart attack. Either one would warrant emergency surgery.