After using that line of argument to great effect throughout the year against Jeb Bush, the former governor of Florida, Mr. Trump has begun to question whether Mrs. Clinton has the “stamina” to be president.

In an appearance on ABC’s “This Week,” Mr. Trump said that the president must deal with “very, very tough people.”

“They only understand strength,” Mr. Trump said. “They don’t understand weakness. Somebody like Jeb, and others that are running against me — and by the way, Hillary is another one. I mean, Hillary is a person who doesn’t have the strength or the stamina, in my opinion, to be president. She doesn’t have strength or stamina. She’s not a strong enough person to be president.”