Bush, the former Florida governor, vowed Friday not to support Trump or the Democratic frontrunner should the two face off in the general election.

"In November, I will not vote for Donald Trump or Hillary Clinton, but I will support principled conservatives at the state and federal levels, just as I have done my entire life," Bush wrote on the social media site Facebook.

Bush added that Trump has not demonstrated the temperament or strength of character needed to serve as president. Of Clinton, he says the former secretary of state is untrustworthy and he expressed pessimism that either candidate would be a good leader.

The withering criticism from Bush comes just days after other big-name Republicans piled on the flamboyant real estate mogul and former reality TV star. Republican House Speaker Paul Ryan and South Carolina Sen. Lindsey Graham both said this week they won't support Trump.

Trump's reaction to the open hostility from within his own party was characteristically cavalier.

"With millions of people coming into the party, obviously I'm saying the right thing," Trump said on the right-wing Fox News Channel. "I mean, he talks about unity, but what is this?"

Democrats are generally avoiding wading into the controversy of Republican infighting, letting the party's leaders tear each other apart.

But US President Barack Obama did say when questioned that the tone of the race needed to get serious.

"This is not entertainment. This is not a reality show," and candidates need to show they have the qualities to lead the world's strongest nation, the president said.

jar/bw (AP, Reuters)