Washington: Former President George W. Bush has delivered a scathing warning about Donald Trump, saying his "America first" philosophy portends a dangerous inward turn that is eroding democracy at home and threatening stability around the world.

"The health of the democratic spirit is at issue," the 43rd president said during a speech in New York on Thursday, local time. "And the renewal of that spirit is the urgent task at hand.

"Since World War II, America has encouraged and benefited from the global advance of the free markets, from the strength of democratic alliance and from the advance of free societies," Bush said.

"Bullying and prejudice in our public life sets a national tone, provides permission for cruelty and bigotry, and compromises the moral education of children."