MIAMI - The Republican front-runner for the presidential nomination, Donald Trump, is not prepared to be the nation's commander in chief, because he's "uninformed" on major issues facing the U.S, especially defense and foreign affairs, rival Jeb Bush said on Sunday.

"I have great doubts about Donald Trump's ability to be commander in chief," Bush said on CBS's Face the Nation. "If you listen to him talk, it's kind of scary to be honest with you, because he's not a serious candidate."

Bush said Trump has offered conflicting opinions on his strategy to deal with Syria and the Islamic State group.

Trump "said we had no interest in being involved in Syria. And then he said let the Russians take out ISIS. And then he said let ISIS take out Assad. I just think he's uninformed," Bush said, using an alternative acronym for the Islamic State group.

Trump is "all over the map, misinformed at best and praying on people's fears at worst," Bush said.

GOP presidential candidate John Kasich, speaking on ABC's This Week, called Trump a "very divisive" candidate and, like Bush, predicted he would not be the eventual Republican nominee.

Other GOP presidential candidates also swiped at Trump.

On Fox News Sunday, Carly Fiorina said Trump's game plan is to say something "insulting, offensive, outrageous," so that the "media pays attention," and then he "claims we all misunderstood him."

"This is the pattern perhaps of an entertainer," she said. "It's certainly not a pattern of a leader."

"Donald Trump only feels big when he's trying to make everyone else look small," she said. "Of course, in the end, he looks the smallest of all."

Also Sunday, Trump canceled a Monday news conference in which his campaign had said he would be endorsed by as many as 100 black evangelical religious leaders. Many of those invited to the event say they had no intention of endorsing the businessman.

Trump campaign spokeswoman Hope Hicks said in an email that Trump still would be holding a private meeting with the group on Monday before departing for a rally in Georgia.