Republican presidential front-runner Donald TrumpDonald John TrumpO’Malley tells Dems not to fear Trump Right way and wrong way Five things to know about the elephant trophies controversy MORE on Wednesday fired back at Democratic rival Hillary ClintonHillary Diane Rodham ClintonO’Malley tells Dems not to fear Trump FBI informant gathered years of evidence on Russian push for US nuclear fuel deals, including Uranium One, memos show Pelosi blasts California Republicans for supporting tax bill MORE for suggesting that the billionaire would embarrass the U.S. around the world if he's elected president.

"I think she's an embarrassment to our country," Trump said during a phone interview on CNN's "New Day." "She's under federal investigation. She doesn't have the strength or the stamina to be president."

"She talks about defeating our enemies. Well, where had she been for the last year? We can't even beat ISIS," Trump said, referring to the Islamic State in Iraq and Syria. "She wouldn't know how to defeat the enemy."

Trump, and to a greater extent Clinton, moved closer to their party's respective nominations with wins in several states on Tuesday.

Clinton late Tuesday tweaked her Republican opponent, criticizing his positions on waterboarding and suggesting he'd be a nightmare for the U.S. diplomatically.

"Our commander in chief has to be able to defend our country, not embarrass it," Clinton said during her speech claiming victory in at least four states in the Democratic race.

The former secretary of State ripped Trump's plans for "rounding up 12 million immigrants, banning all Muslims from entering the United States" and for embracing "torture."