Donald Trump has begun the first full week of the US general election campaign with his strategy of saying the politically unimaginable facing its greatest test to date.

Mr Trump broke a major American political and societal taboo over the weekend when he engaged in a feud with Khizr and Ghazala Khan, the bereaved parents of a decorated Muslim captain of the US army who was killed by a suicide bomber in Iraq.

The Republican presidential nominee stoked outrage by implying Ghazala Khan did not speak while standing alongside her husband at the Democratic convention because they are Muslim.

The outcry was bipartisan, leaving Mr Trump largely isolated among his fellow Republicans and potentially risking whatever progress the New Yorker had made in winning over independent voters who will likely decide the election this autumn.

"I am appalled that Donald Trump would disparage them and that he had the gall to compare his own sacrifices to those of a Gold Star family," said the Republican New Hampshire Senator Kelly Ayotte.

Both House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell issued statements praising Capt Humayun Khan, who was awarded a Bronze Star and a Purple Heart after he was killed in Iraq in 2004.

Though neither explicitly mentioned Donald Trump, the congressional leaders pointedly denounced his proposed ban on foreign Muslims entering the country, a policy he has altered in recent weeks but nevertheless one that returned to the centre of the campaign debate via his attacks on the Khan family.

For the second time in a week, Mr Trump's running mate, Indiana governor Mike Pence, issued a statement that appeared designed to put some space between the two men.

The father of a US Marine, Mr Pence said that he and Mr Trump believe Capt Khan is a hero and his family "should be cherished by every American".

Last week, Mr Pence said Russia would face "serious consequences" for meddling in US elections at roughly the same time as Mr Trump appeared to encourage it, telling reporters he would welcome Russia unearthing emails that Democratic nominee Hillary Clinton deleted from the private servers she used during her time as US secretary of state.

At last week's Democratic convention, the Pakistan-born Khizr Khan told his son's story, questioned whether Mr Trump had ever read the US Constitution and said: "You have sacrificed nothing."

During the speech, Ghazala Khan stood quietly by his side.

Mr Trump responded in a TV interview, saying: "If you look at his wife, she was standing there. She had nothing to say. She probably, maybe she wasn't allowed to have anything to say."

Ghazala Khan wrote in Sunday's Washington Post that she did not speak because talking about her son's death remains difficult for her.

"Every day, whenever I pray, I have to pray for him, and I cry. The place that emptied will always be empty," she wrote.

Although Mr Trump tried several times to deflect the criticism, he refused to back down from his initial attack.

"Am I not allowed to respond?" Trump tweeted on Sunday morning. "Hillary voted for the Iraq war, not me!"

His tweet followed a Saturday night statement where Mr Trump described Humayun Khan as "a hero," but said his father had "no right" to "say many other inaccurate things".

A spokesman for Mr Trump said that a biased media was at work, framing the issue as one of "Trump vs Khan", suggesting that the real issue was a "Trump vs Clinton" battle over fighting terrorism.

He added that Mrs Clinton camp's was stirring controversy to distract from the Democratic nominee's weaknesses on national security issues.

Mrs Clinton reached out to Republicans over Mr Trump's comments, telling an audience in Ohio: "One doesn't know where the bottom is. It's hard to imagine anyone who has ever run to be president of the United States saying any of what he's said.

"This is a time to pick country over party."