Former White House adviser Steve Bannon has described former US president George W Bush as bumbling and inept, criticising his "destructive" presidency from 2000-2008.
Mr Bannon's scathing remarks were a retort to a speech by Mr Bush in New York earlier this week, in which the 43rd president denounced bigotry in Trump-era American politics and warned that the rise of "nativism", isolationism and conspiracy theories have clouded the nation's true identity.
But President Donald Trump's former adviser, speaking to a capacity crowd at a California Republican Party convention, said Mr Bush had embarrassed himself and did not know what he was talking about.
The remarks came during a speech thick with attacks on the Washington status quo, echoing his call for an "open revolt" against establishment Republicans.
Mr Bannon said Mr Bush has no idea whether "he is coming or going, just like it was when he was president".
"There has not been a more destructive presidency than George Bush's," Mr Bannon added, as boos could be heard in the crowd at the mention of the former president's name.
Mr Bannon also called the "permanent political class" one of the great dangers faced by the country.
A small group of protesters gathered outside the hotel where Mr Bannon spoke, chanting and waving signs - one displaying a Nazi swastika.
The protesters were kept behind steel barricades on a plaza across an entrance road at the hotel, largely out of view of people entering for the event. No arrests were reported.
Mr Bannon also took aim at Silicon Valley and its "lords of technology", predicting that tech leaders and progressives in the state would try to secede from the union in 10 to 15 years.
He called the threat to break up the nation a "living problem".
Mr Bannon also tried to cheer long-suffering California Republicans, in a state that Donald Trump lost by more than four million votes and where Republicans have become largely irrelevant in state politics.
In Orange County, where the convention was held, several Republican House members are trying to hold onto their seats in districts carried by Hillary Clinton in the 2016 presidential contest.
"You've got everything you need to win," he told them.
Mr Bannon ended his speech with a standing ovation.