First there was “low energy.” Now increasingly it’s “no stamina.”

Donald J. Trump dispatched his Republican primary opponents with the power of branding, most notably billing Jeb Bush as a weak-wattage candidate who lacked the pizazz to be president. As Mr. Trump faces an uphill battle in the general election campaign against Hillary Clinton, the Republican presidential nominee is trying to reprise the winning formula that got him this far, slipping it into rallies and speeches that his opponent appears too tired to lead the country.

In a prominent national security address on Monday, Mr. Trump could not have been more explicit when he said that Mrs. Clinton “lacks the mental and physical stamina to take on ISIS, and all the many adversaries we face.”

The remark drew chortles on social media, with some pointing out that the commander in chief is not needed on the front lines against the Islamic State these days and others recalling that Mr. Trump received a draft deferment during the Vietnam War because of bone spurs in his feet. Women on Twitter said the comment reeked of sexism. The former tennis star Martina Navratilova wondered if Mr. Trump was giving himself too much credit.