President Barack Obama said that the Republican presidential nominee Donald Trump is "unfit to serve as president".
He said that while he had had policy differences with previous Republican candidates, he thought they would be able to to the job. In contrast, he said Trump, the candidate for this year's election, is "woefully unprepared".
It follow Trump's attacks on a Muslim family whose son was killed serving in the US military in Iraq after they spoke out against him at last week's Democratic National Convention. Obama also criticised the real estate mogul and reality TV star's comments on foreign affairs.
"The notion that he would attack a Gold Star family that made such extraordinary sacrifices on behalf of our country, the fact that he doesn't appear to have basic knowledge around critical issues in Europe, in the Middle East, in Asia, means that he is woefully unprepared to do this job," the president said at the White House press briefing.
Senator John McCain, Obama's 2008 opponent, is among Republicans who have criticised Trump for his attacks on the family of US Army Capt Humayun Khan, who was killed in 2004 by a car bomb in Iraq aged 24.
At the convention the soldier's father, Khizr Khan, had denounced Trump for his pledge to ban Muslims from entering the United States if elected president.
Obama said that Republican denunciations of Trump "ring hollow", as the party's senior officials continue to back his bid for the presidency.
"There has to come a point at which you say enough," he said. "The question that I think that they have to ask themselves is, 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? This isn't a situation where you have an episodic gaffe. This is daily and weekly where they are distancing themselves from statements he's making."