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Donald J. Trump has, over the last few days, been road-testing a new criticism of Hillary Rodham Clinton — that she is too low-energy to be president.

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.”

Mr. Trump used the line last week in Massachusetts as well, weeks after Mrs. Clinton’s marathon testimony before the House committee investigating the 2012 terrorist attacks on the United States diplomatic compound in Benghazi, Libya.

An aide to Mrs. Clinton did not respond to an email on Sunday seeking comment.