Republican presidential candidate Donald Trump is seen on day three of the Republican National Convention at the Quicken Loans Arena in Cleveland, Ohio on July 20, 2016
Washington (AFP) - Veterans groups and Republicans Monday savaged Donald Trump's war of words with the parents of a slain Muslim American soldier, the latest example of the presidential nominee making statements considered beyond the pale.
The feud has shaken the US presidential campaign, becoming a flash point 99 days before the November election that pits the Republican against Democrat Hillary Clinton, whom Trump on Monday called "the devil."
Criticism has poured in from outraged veterans groups, relatives of soldiers killed in action and even President Barack Obama.
The episode has dominated US headlines since last Thursday, when Pakistani immigrant Khizr Khan galvanized the Democratic National Convention with a tribute to his dead son in which he rebuked the Republican nominee for having "sacrificed nothing" for the country.
In an interview aired on ABC Sunday, Trump insisted he had made "a lot of sacrifices" while suggesting that Khan's wife, who stood silent on the convention stage as her husband spoke, had not been allowed to talk.
On Monday, Trump renewed his assault, tweeting: "Mr. Khan, who does not know me, viciously attacked me from the stage of the DNC and is now all over TV doing the same - Nice!"
The families of 23 other slain US soldiers berated Trump for remarks they called "repugnant and personally offensive."
"We feel we must speak out and demand you apologize to the Khans, to all Gold Star families, and to all Americans for your offensive, and frankly anti-American, comments," they said in an open letter.
Army captain Humayun Khan was killed by a suicide bomber in Iraq in 2004.
Veterans of Foreign Wars, the largest US war veterans group, heaped scorn on Trump's "out of bounds" criticism of a fallen soldier's mother.
"Election year or not, the VFW will not tolerate anyone berating a Gold Star family member for exercising his or her right of speech or expression," VFW leader Brian Duffy said.
Trump broke precedent again on Monday, lambasting Clinton's nomination rival Bernie Sanders for giving in to her nomination, saying the Vermont senator "made a deal with the devil. She's the devil. He made a deal with the devil. It's true."
- Republican backlash -
Trump's running mate Mike Pence attempted to walk back the billionaire's war of words with Khan Monday, telling an angry Air Force mother whose son is serving in the military that "Captain Khan is an American hero."
"We honor him and honor his family as we do all Gold Star families," Pence told the woman in Carson City, Nevada, after she asked the vice presidential nominee how he could tolerate Trump's "disrespect."
Few high-profile Republicans have backed Trump in his battle with the Khans.
The mogul's sustained hostility toward the couple -- alarming partly because criticism of Gold Star families has traditionally been off-limits in American political discourse -- incensed Senator John McCain, a former prisoner of war once mocked by Trump for being captured in Vietnam.
"I cannot emphasize enough how deeply I disagree with Mr Trump's statement," McCain said in a lengthy and scathing statement.
"While our party has bestowed upon him the nomination, it is not accompanied by unfettered license to defame those who are the best among us," he added.
Former 2016 White House candidate Jeb Bush blasted Trump's statements as "incredibly disrespectful," while the chairman of the Republican National Committee signaled he, too, was shocked by the billionaire.
"I think this family should be off limits," Reince Priebus told CNN.
- Ultimate sacrifice -
With the two sides slinging criticism, Khan accused Trump of Islamophobia, assailing his "ignorance and arrogance," expressing exasperation on NBC that the provocative billionaire "can get up and malign the entire nation."
Obama issued his own thinly veiled attack on Trump, telling a group of disabled veterans he was tired of some people "trash-talking" America's military and troops.
"No one has given more for our freedom and our security than our Gold Star families," the president said.
Ghazala Khan, whom Trump questioned for standing quietly as her husband talked about their son, spoke out in Monday's Washington Post.
"Without saying a thing, all the world, all America, felt my pain," she wrote.
Trump scrambled to pivot away from taking on military families, tweeting Monday that the issue was "not about Mr. Khan, who is all over the place doing interviews, but rather RADICAL ISLAMIC TERRORISM and the US. Get smart!"
For months, Trump warned that the Democratic primary system was "rigged" against Sanders, but on Monday, he stoked distrust about the larger election.
"I am afraid the election is going to be rigged. I have to be honest," he told a rally in Columbus, Ohio.
A CNN poll Monday showed Clinton with a nine point lead over Trump, 52-43, reflecting a seven-point bounce in support after her party's convention.
Meanwhile, a CBS News Poll found that six in 10 voters believe Trump is not prepared to be president, while 60 percent believe Clinton is prepared for the job.
Clinton on Monday welcomed the endorsement of American billionaire and investment pioneer Warren Buffett, who joined her onstage in Omaha, Nebraska to berate Trump for not releasing his tax returns because he is being audited.
"You're only afraid if you got something to be afraid about," Buffet told the crowd.
"He's not afraid of the IRS. He's afraid because of you."