Donald Trump told Fox News sit-in host Mike Huckabee that he has no choice than to serve as the Republican nominee through the election in November.

"It's set up and we have to do it. It's so important. We have 90 days left and I wanna maximize the time," Trump told the former Arkansas governor late Friday on Hannity. "I just feel that I have no choice. I have to really go full blast and get this thing done because we will make America great again."

Trump shied away from explaining if he has the stamina to serve as president. Instead, he argued for the importance of the stamina a generic political leader needs to succeed.

"If you don't have the stamina, if you don't have the whatever that is you have to have, the country's in trouble — not you — the country's in trouble," Trump added. "I've always said Hillary doesn't have the stamina, but you just need a great deal of strength for us to come out a winner."

Evan McMullin, a former CIA agent who announced his independent presidential bid earlier this week, chided Trump for lacking the energy to fight for America longterm.

"Look, Donald Trump is defeating himself. He is ensuring that Hillary Clinton is elected," McMullin said. "It's clear to me that he is a weak candidate. Trump is weak and, in fact, I question whether he has the stamina to complete the race."

Trump has previously cited his decades of work building businesses and real estate empire as evidence of his endurance and stamina. But Democratic nominee Hillary Clinton appears to be spending more money and time reaching voters across the country in ads and rallies, which would suggest Trump's campaign is not, perhaps, up to it after all.