A bipartisan constellation of decorated combat veterans, members of Congress and family members of slain soldiers admonished Donald Trump on Monday for criticizing the Muslim American parents of an Army officer killed in Iraq, threatening to undermine Trump’s support among core Republican voters.

The condemnations by Senate Armed Services Committee Chairman John McCain (R-Ariz.), the Veterans of Foreign Wars, and dozens of veterans and family members of those killed in the line of duty served as the most forceful rebuke yet of the mogul’s comments and his anti-Muslim rhetoric.

The critiques lobbed at Trump on Monday were the latest turns in a bitter exchange that has dominated the presidential race since the close of the Democratic National Convention on Thursday in Philadelphia. It threatens to hurt Trump’s standing among voters he has been aggressively pursuing: those who aren’t fans of Democrat Hillary Clinton and who hold doubts about her record on national security. The standoff has also frayed Trump’s already delicate alliance with GOP leaders.

[‘We’ve never been challenged this way’: Military support groups demand respect for Khan family]

Trump did not address the controversy directly during a campaign stop in Columbus, Ohio, on Monday afternoon. But he signaled on Twitter earlier in the day that he was not backing down from his criticism of Khizr and Ghazala Khan, whose son Humayun Khan, an Army captain, was killed by a car bomber in Iraq in 2004. Trump said Khizr Khan had “no right” to assail him as he did in a speech at the Democratic convention Thursday.

Both Republicans and Democrats are publicly responding to the row between GOP presidential nominee Donald Trump and Khizr Khan. Khan's son was killed while serving in Iraq. Here's what politicians from both parties said. (Monica Akhtar/The Washington Post)

McCain, a respected figure on national security issues in the Republican Party, issued a written statement sternly reprimanding Trump.

“In recent days, Donald Trump disparaged a fallen soldier’s parents,” said McCain, who was taken prisoner during the Vietnam War. “He has suggested that the likes of their son should not be allowed in the United States — to say nothing of entering its service. I cannot emphasize enough how deeply I disagree with Mr. Trump’s statement. I hope Americans understand that the remarks do not represent the views of our Republican Party, its officers, or candidates.”

[GOP dismay grows as Trump stands his ground]

McCain, who has tangled with Trump before, most notably after Trump said last year that McCain was not a war hero because he had been “captured,” added: “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.”

Aside from McCain, a bipartisan coalition of veterans, family members of military personnel killed in the line of duty, a veteran serving in Congress and an ex-diplomat sent a letter to Trump calling his criticism of the Khans an affront to each of them. It also called for him to apologize.

“Your statements are unacceptable, especially from someone seeking to serve as Commander in Chief,” the letter said. “The Khans’ sacrifice has earned them the right to ask hard questions of those seeking elected office.”

[Humayun Khan’s grave becomes a shrine in the wake of his father’s speech]

1 of 14 Full Screen Autoplay Close Skip Ad × These Republicans refuse to vote for Donald Trump View Photos And they’ll tell you why. Caption And they’ll tell you why. Former Secretary of State Colin Powell “General Powell said at a meeting of the Long Island Association that he would be voting for Hillary Clinton,” a spokeswoman confirmed Oct. 25. Powell added in an interview that he picked Clinton “because I think she’s qualified, and the other gentleman is not qualified.” Nikki Kahn/The Washington Post Buy Photo Wait 1 second to continue.

The list of signatories includes highly decorated combat veterans including Medal of Honor recipient Dakota Meyer, who was a supporter of Sen. Ted Cruz’s presidential campaign. Rep. Seth Moulton (D-Mass.), who served in Iraq as a Marine Corps infantry officer, is also on the list.

Organizers said they plan to add many more signatures by opening up the letter online for broader national participation.

Brian Duffy, the recently elected commander in chief of the Veterans of Foreign Wars, released a statement saying that the organization “will not tolerate anyone berating a Gold Star family member for exercising his or her right of speech or expression.”

Duffy added that “there are certain sacrosanct subjects that no amount of wordsmithing can repair once crossed.”

While Trump pledged to take good care of veterans at his campaign stop in Columbus, he steered clear of the firestorm over his comments about the Khans. Attendees had their own opinions on the matter.

Josh Smith, 53, of Keystone Heights, Fla., said that he served in the Navy for 20 years and that he did not find Trump’s comments about the Khans to be distasteful. “It was bad that their son died, but to put that man on stage to say all those things was ridiculous,” he said.

Smith said that when Trump responded to the Khans’ remarks, “he was just making a comment back to someone who attacked him. He wasn’t being offensive.”

Larry Robinson, 71, of Columbus, who said he served in the Army during the Vietnam War, believes that Trump has a lot of empathy for veterans.

On Twitter on Monday morning, Trump lashed out again at Khizr Khan and the media. He argued that “radical Islamic terrorism,” not Khan, should be the focus of the exchange — just minutes after slamming Khan.

“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!” Trump said in his initial tweet.

Khizr and Ghazala Khan participated in a round of television interviews Monday in which they slammed Trump.

“This candidate amazes me — his ignorance,” Khizr Khan said on NBC’s “Today” show. “He can get up and malign the entire nation — the religions, the communities, the minorities, the judges. And yet, a private citizen in this political process, in his candidacy for the stewardship for this country — I cannot say what I feel? That proves the point: He has not read the Constitution of this country.”

Ghazala Khan, who teared up during at least one TV interview, rejected Trump’s suggestion during an interview with ABC News that she may not have been “allowed” to speak during her convention appearance. She stood at her husband’s side as he spoke.

[In Trump clash with Khans, new hints of cultural and political rift]

“It doesn’t have to do anything with my religion,” she said on “Today.” She wrote in a Washington Post op-ed over the weekend that she is still experiencing raw emotions about her son’s death and could not bring herself to speak at the convention.

Clinton has said that the Khan family paid the “ultimate sacrifice” for their country. Ahead of her rally in Omaha, the controversy was a top-of-mind issue.

“We’re both grandmothers; we both think it’s terrible,” said Jan Gleason, 79, as she stood with a friend waiting for Clinton to arrive.

“It’s just a person who served and died for our country. He made fun of his mother,” added Sharon Heck, 72. “It’s just sad.”

Speaking to the Disabled American Veterans’ annual convention in Atlanta, President Obama did not mention Khan’s parents or their speech. But he said he had asked to be introduced at the Democratic convention last week by Sharon Belkofer, whose son was killed in combat. And he called on Americans to support those who have lost loved ones to war. “We have to do everything we can for those families, and honor them, and be humbled by them,” Obama said.

Some Trump allies fought back hard Monday against the criticism the mogul has faced over his remarks about the Khans.

Corey Lewandowski, Trump’s former campaign manager, argued that the Khans’ son would still be alive if Trump had been president.

“Their son is a hero. And every person who has ever died fighting for our country and their families are heroes,” Lewandowski said on CNN, which employs him as a paid contributor. “The difference is, we’ve got 7,000 soldiers who died, $6 trillion wasted in wars overseas, and if Donald Trump was the president, we would never have had, and Captain Khan would be alive today.”

Trump regularly casts himself as an early critic of the Iraq War. The Post’s Fact Checker found that there is “no sign that Trump opposed the invasion or was vocal about it prior to the invasion.”

Lewandowski was fired by Trump in June.

Former New York mayor Rudolph W. Giuliani said McCain’s response to Trump was probably motivated by the longtime senator’s anxiety over his reelection race in Arizona.

“I think you’re hearing a guy who’s worried about whether he can be reelected in Arizona,” Giuliani said in an interview with The Post. “John wouldn’t be saying this if he were running two years from now. He’d just keep his mouth shut.”

James Waters, a former George W. Bush administration White House aide and Navy SEAL, was among the 40 who signed the bipartisan letter to Trump. He is a Republican, but he is not voting for his party’s nominee.

“I think this situation speaks quite well to Trump’s blatant lack of fitness for office,” Waters said.

Kathy Lynn Gray in Columbus; Greg Jaffe in Atlanta; Katie Zezima, Dan Lamothe and Robert Costa in Washington; and Abby Phillip in Omaha contributed to this report.