Vice President Kamala Harris blasted former President Donald Trump for becoming “exhausted” by his campaign schedule on Friday, arguing that anyone without the stamina to run a campaign can’t be an effective president.

“He is ducking debates and canceling interviews. His own campaign team recently said it’s because of exhaustion,” Harris said at a rally in Michigan, in her most direct shot at her opponent’s health yet. “If you’re exhausted on the campaign trail, it raises real questions about whether you are fit for the toughest job in the world.”

The attacks followed questions on whether Trump is ramping down his campaign commitments due to physical or mental limitations. The candidate pulled out of an NRA rally in Georgia on Thursday, following a raft of media cancellations.

The Grand Rapids speech came a day after Harris made three stops in Wisconsin.

Harris became the Democratic nominee for president following President Joe Biden’s exit amid concerns about his age. Biden was then the oldest candidate for president in U.S. history. Trump would be the oldest president ever elected were he to win in November.

Bucking questions about his own cognitive decline, Trump responded to the jab by denying reports that he was tuckered.

“Tell me when you’ve seen me take even a little bit of a rest, not only am I not– I’m not even tired. I’m really exhilarated,” Trump said on Friday. “She’s not a smart person.”