WASHINGTON — John F. Kelly, the White House chief of staff, delivered a searing, personal defense of President Trump’s phone call this week to the widow of a slain Army soldier, describing on Thursday the trauma of learning about his own son’s death in Afghanistan and calling the criticism of Mr. Trump’s condolences unfair.

In a public relations mission that was part emotional catharsis and part political attack, Mr. Kelly lashed out at Representative Frederica S. Wilson, Democrat of Florida, for publicizing the call between Mr. Trump and Myeshia Johnson, whose husband, Sgt. La David T. Johnson, was one of four American soldiers killed in an Oct. 4 ambush in Niger.

Mr. Kelly accused Ms. Wilson — who was in a car with Ms. Johnson when Mr. Trump called and is a longtime family friend — of being a publicity-seeking opportunist. He said that the congresswoman’s willingness to breach the confidentiality of Mr. Trump’s words is evidence of a broader decline in the values of an American society that no longer treats women, religion, “life” or Gold Star families as sacred.

“It stuns me that a member of Congress would have listened in on that conversation — absolutely stuns me,” Mr. Kelly said during a somber, 18-minute appearance in the White House briefing room. He said that he was so upset by Ms. Wilson’s appearances on TV news shows that he had to collect his thoughts by walking through the graves at Arlington National Cemetery for more than an hour.