Hillary Clinton has accused Donald Trump of insulting America's veterans and pressing dangerous military plans around the globe, seeking to undercut his appeal to service families in southern voting battlegrounds.
Meanwhile, Mr Trump declared "our country is going to hell" because of policies Ms Clinton would make worse.
Ms Clinton, addressing supporters in Florida, warned that Mr Trump would lead the nation back to war in the Middle East, and to military veterans and their families, she pointed anew to his summertime dust-up with the Muslim parents of a killed American soldier.
"His whole campaign has been one long insult to all those who have worn the uniform," the Democratic nominee said at the University of South Florida in Tampa.
Republican Mr Trump, trying emphasise his military support, released a letter from 88 retired generals and admirals citing an urgent need for a "course correction" in America's national security policy. It was aimed at rebutting Ms Clinton's arguments that she would be best positioned to lead the military and reassuring Republicans who have openly worried that his provocative statements might undermine US alliances.
"We believe that such a change can only be made by someone who has not been deeply involved with, and substantially responsible for, the hollowing out of our military and the burgeoning threats facing our country around the world," the military leaders wrote. "For this reason, we support Donald Trump's candidacy to be our next commander in chief."
Ms Clinton pushed back, saying Mr Trump has lagged in securing key military supporters compared with past Republican nominees including John McCain and Mitt Romney. She pointed to her endorsements from retired Marine General John Allen, who blasted Mr Trump at the Democratic National Committee, and former CIA deputy director Mike Morell.
"They know they can count on me to be the kind of commander in chief who will protect our country and our troops, and they know they cannot count on Donald Trump," Ms Clinton said en route to Florida. "They view him as a danger and a risk."
The conflicting messages came as the candidates prepared to appear at an MSNBC forum on national security. While they will appear separately and not be on stage at the same time, it could serve as a warm-up to their highly anticipated first presidential debate on September 26 in New York.
Campaigning in Virginia Beach, Mr Trump vowed to take aggressive action to help veterans at home and confront threats abroad including acts of terrorism from the Islamic State group. He was questioned by retired General Michael Flynn, the former director of the Defence Intelligence Agency who is a strong supporter.
"We are going to solve the Isis problem," Mr Trump said. "But we have to get back to building our country, because our country is going to hell."
Donald Trump has a problem looking at someone different from himself and actually seeing them. pic.twitter.com/LddoXYkBsf— Hillary Clinton (@HillaryClinton) September 6, 2016
He promised to fix problems at the Veterans Administration, which has grappled with patient care mismanagement during the Obama administration. Until those problems are resolved, he said he would allow veterans to seek treatment at private doctors or hospitals free of charge.
"Your government is going to pay your bill," he pledged.
Ms Clinton's message was amplified by her running mate, Virginia senator Tim Kaine, who promoted his own foreign policy credentials in a speech in Wilmington, North Carolina.
He said Mr Trump is misleading voters on his foreign policy views, asserting he was against invading Iraq despite statements to several news outlets at the time indicating otherwise.
Mr Kaine, who noted his own son's service in the Marine Corps, is a member of the Senate Armed Services and Foreign Relations committees.
Meanwhile, Ms Clinton's campaign released a new television ad entitled Sacrifice, which shows military veterans watching some of the New York businessman's more provocative statements.
The spot includes clips of Mr Trump claiming to know more about Islamic State than military generals, and his criticism of Mr McCain, the Republican senator from Arizona and a former prisoner of war. The ad, which features former Georgia senator Max Cleland, a triple-amputee who served in Vietnam, also keys on Mr Trump's assertion that he has sacrificed a lot compared with families who have lost loved ones in conflict.
Republicans have also questioned Mr Trump's capacity to serve as commander in chief. Dozens of national security leaders released a letter last month warning that he would risk the nation's "national security and well-being".