Obama in blistering attack on 'woefully unprepared' Trump
US president Barack Obama has declared Donald Trump is "unfit" and "woefully unprepared" to serve in the White House.
In a searing denouncement, Mr Obama challenged top Republicans to drop their support for their party's nominee, declaring: "There has to come a point at which you say, 'enough'."
The outgoing president's blistering critique of his potential successor followed Mr Trump's criticism of an American Muslim family whose son, a captain in the US Army, was killed in Iraq.
A growing number of Republicans have disavowed Mr Trump's comments, but most of those who have endorsed him are sticking by that stance.
At a White House news conference, Mr Obama said: "If you are repeatedly having to say, in very strong terms, that what he has said is unacceptable, why are you still endorsing him?
"What does this say about your party that this is your standard-bearer?"
The US president said his opposition to Donald Trump is about more than policy differences. He said that while he disagreed with his Republican opponents in the 2008 and 2012 elections, he never thought they were unfit to do the job.
Mr Obama - who is enjoying heightened popularity in his eighth and final year in office - has made clear he plans to be an active player in the White House race, campaigning around the country for Democratic nominee Hillary Clinton.
The president and first lady Michelle Obama spoke at last week's Democratic convention in Philadelphia, where Khizr and Ghazala Khan also made an appearance.
Mr Trump responded with a statement that summarised the points he makes in his stump speeches: "Obama-Clinton have single-handedly destabilised the Middle East ... released criminal aliens into our country who killed one innocent American after another ... produced the worst recovery since the Great Depression (and) shipped millions of our best jobs overseas."
Last week, Khizr Khan criticised Mr Trump's call for a temporary ban on Muslims coming to the United States and challenged whether he had read the American Constitution.
Mr Trump has questioned why Ghazala Khan did not speak, implying her religion prevented her from doing so, and has said he was "viciously attacked" by Khizr Khan.
Mr Trump spent the days after winning the Republican nomination criticising a US district court judge's Mexican heritage. The morning after accepting the Republican nomination at the party's convention, he re-opened months-old grievances with primary rival Ted Cruz.
Those who have worked with Mr Trump say that in private meetings, he can often appear amenable to putting a controversy aside. But the businessman can quickly be drawn back in by an interview, especially if he believes he has already answered the question, or if he grows irritated by commentary on cable television.
Khizr Khan delivered an emotional address at last week's Democratic convention, with his wife standing by his side. The Pakistan-born Khan told the story of his son, US army Capt Humayun Khan, who was posthumously awarded a Bronze Star and Purple Heart after his death in 2004.
Mr Trump's unwillingness to let the matter subside sparked outrage from several Republicans.
Arizona Senator John McCain, a former prisoner of war, said Trump did not have "unfettered licence to defame those who are the best among us". #
Mike Coffman, a vulnerable Republican in a competitive Colorado district, said he was "deeply offended when Donald Trump fails to honour the sacrifices of all of our brave soldiers who were lost in that war".
Missouri Senator Roy Blunt said the Khans "deserve to be heard and respected".
Mr Blunt added: "My advice to Donald Trump has been and will continue to be to focus on jobs and national security and stop responding to every criticism whether it's from a grieving family or Hillary Clinton."
But when asked about Khizr Khan on Fox News Channel's "Hannity," Trump responded, "His son died 12 years ago ... If I were president, his son wouldn't have died, because I wouldn't have been in the war, if I was president back then."