Former US Republican President George Bush Sr has confirmed he voted for Democrat Hillary Clinton in the 2016 presidential election, labelling Donald Trump a "blowhard".

His son, George W Bush, said he worries that "I will be the last Republican president", even though President Trump is a Republican.

"This guy doesn't know what it means to be president," the younger man said.

The pair's comments come from a new book, titled The Last Republicans.

Preview excerpts from the book were published by US media outlets.