In a searing and virtually unprecedented presidential rebuke, Barack Obama declared embattled Republican White House nominee Donald Trump "unfit" to be president Tuesday and called on party leaders to disown him.

Obama piled on as Trump's campaign reeled from multiple self-inflicted scandals, calling the 70-year-old mogul "woefully unprepared" and "unfit to serve as president."

"He keeps proving it," said Obama, standing alongside the prime minister of Singapore and casting aside any pretense of domestic unity.

In recent days, Trump has criticized Muslims, babies, firefighters and the military, prompting his wincing Republican backers to issue awkward denunciations.

Congressman Richard Hanna went one step further, becoming the first Republican lawmaker to say he will vote for Trump's opponent, Democrat Hillary Clinton, in November.

"I find Trump deeply flawed in endless ways," Hanna wrote in a newspaper editorial announcing his decision.

Obama turned up the heat on Republicans who appear increasingly ill at ease withTrump buthave not withdrawn their endorsement.

"This isn't a situation where you have an episodic gaffe," Obama said. "This is daily and weekly where they are distancing themselves from statements he's making."

"There has to be a point in which you say: 'This is not somebody I can support for president of the United States, even if he purports to be a member of my party.'"

"There has to come a point at which you say 'enough'," he said.

"The alternative is that the entire party, the Republican Party, effectively endorses and validates the positions that are being articulated by Mr Trump."


Trump hits back

Leaders like House Speaker Paul Ryan and Senator John McCain may have been given further pause by Trump's refusal to reciprocate their endorsements.

Trump won the Republican primary handily, but is trailing Clinton in general election polls by around four percentage points.

Obama has already endorsed his fellow Democrat and has repeatedly pilloried Trump's populism.

But his comments in the East Room of the White House -- where Abraham Lincoln lay in state and Theodore Roosevelt today casts a painted gaze -- are a significant and highly personal escalation of presidential rhetoric.

Last week, Obama addressed the Democratic convention in Philadelphia and painted this election as a choice not between a Democrat and Republican, but a Democrat and a demagogue who threatens democracy.

"There have been Republican presidents with whom I've disagreed with, but I didn't have a doubt that they could function as president," Obama said Tuesday.

Turning to his 2012 and 2008 election opponents, Obama said "Mitt Romney and John McCain were wrong on certain policy issues, but I never thought that they couldn't do the job."

Trumphitback at Obama in a written statement, describing his two terms in office as an example of "failed leadership."


Gold star fight

Obama's comments came amid a roiling war of words between Trump and the father of a slain US soldier who rebuked the Republican nominee as having "sacrificed nothing."

"The notion that he would attack a Gold Star family that made 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's woefully unprepared to do this job," said Obama.

On the campaign trail on Tuesday, Trump further dropped jaws by telling a mother and her crying baby to leave a rally and saying he "always wanted to get a Purple Heart," after being given one by a military veteran who supports him.

The military honor is given by a sitting president to a member of the armed forces who is killed or wounded in combat.

"This was much easier," Trump remarked.