A war of words between Donald Trump and the parents of a Muslim U.S. soldier killed in Iraq in 2004 is dominating the election campaign, prompting U.S. President Barack Obama to say that as commander-in-chief he’s “tired” of such rhetoric.

Khizr and Ghazala Khan were criticized by the Republican presidential nominee after they appeared at the Democratic National Convention last Thursday and shared the story of their son, U.S. Army Captain Humayun Khan.

The Khans are what is known in the United States as a “Gold Star Family,” meaning they have lost an immediate relative in a military action. The gold star refers to the centerpiece of flags given to these families.

“As commander-in-chief, I’m pretty tired of some folks trash-talking America’s military and troops,” Obama said. “No one – no one – has given more for our freedom and our security than our Gold Star families.”

On Monday, more than 20 Gold Star Families wrote to Trump condemning his treatment of the couple, saying he was “cheapening the sacrifice” of soldiers who have died and demanding that he apologize to the Khans.