Donald Trump is questioning Hillary Clinton's physical fortitude. Trump was asked about facing-off against Hillary Clinton, when he brought up the former secretary of state's "strength" and "stamina."

“I think we'll win it because I think people will see I'm much more competent than she is. I think I'm much smarter than she is. I think I'm much more competent than she is, and I think she's been very weak. I've said it before, I don't think she has the strength or the stamina to be a good president," Trump said.

Trump criticized Clinton's tenure as secretary of state. "She did one thing: she did a lot of traveling ... I think she worked hard. She did a lot of traveling, she was always on planes going back and forth. But she also lied."

Trump made the comments in an interview with Bloomberg TV's With All Due Respect. The interview was conducted by Mark Halperin and John Heilemann. Selected portions of the interview were emailed this morning to members of the press.

Elsewhere in the interview, Trump refused to rule out using nuclear weapons against ISIS. "I'm never going to rule anything out—I wouldn't want to say. Even if I wasn't, I wouldn't want to tell you that because at a minimum, I want them to think maybe we would use them," Trump said.

"We need unpredictability. We don't know who these people are. The fact is, we need unpredictability and when you ask a question like that, it's a very sad thing to have to answer it because the enemy is watching and I have a very good chance of winning and I frankly don't want the enemy to know how I'm thinking. But with that being said, I don't rule out anything."

Trump's interview with Bloomberg TV's With All Due Respect will are this afternoon at 5 p.m.