Clinton defended Khizr Khan and his wife Ghazala, whose son - Army Captain Humayun Khan - was killed in Iraq in 2004.
Clinton denounced Trump's "attacks on distinguished military leaders" and accused him of "insulting the family of a fallen soldier - Captain Khan, an American Muslim who sacrificed his life to protect his unit and other soldiers as a taxi raced toward a base containing a bomb".
After the Khans appeared at the convention, Trump expressed some criticism in an interview with ABC, suggesting that Ghazala Khan was not permitted to speak because she is Muslim. Ghazala Khan has since said that she was unable to speak because she is still overcome with grief over the loss of her son.
Earlier, Clinton called Trump's comments on the Khans part of a pattern.
"One doesn't know where the bottom is," Clinton said in Ashland, Ohio. "It's hard to imagine anyone who has ever run to be president of the United States saying any of what he said."
"And the accumulation of it all is just beyond my comprehension," she added.
But Clinton declined to comment on what Trump's statements say about his character, remarking only that he is "temperamentally unfit and unqualified" to serve as president.
Speaking in Columbus, she returned to Khizr Khan's speech and noted his invocation of the principles of religious liberty "enshrined" in the founding of the country.
"When his father spoke at the convention and pulled out a copy of the Constitution, it was so fitting that that happened in Philadelphia, where our country started 240 years ago," she added.
"George Washington, Thomas Jefferson - they addressed different religions, including Islam, that were present in America from the very beginning.
"I want all of us to stand for freedom and equality and justice and opportunity now and forever."
