How Joe Biden Talks About a Touchy Subject: His Son

The next day, Joe Stutler, 56, who came to see Mr. Biden in Cedar Rapids, Iowa, spoke approvingly of his response. As a cautionary tale, Mr. Stutler cited the discredited attacks on John Kerry’s military record made during the 2004 campaign by the group Swift Boat Veterans for Truth, and said he wanted a candidate “that has the gumption to push back.”

“There are folks that are saying, ‘Oh, well, you know, maybe he should have been a little nicer, blah, blah, blah, blah,’” he said. “No. If somebody’s going to talk smack, call him on it.”

But there is also a delicate line between fiery and bellicose. In response to the tense exchange with the Iowa voter, a spokesman for the Republican National Committee said Mr. Biden “became unglued,” adding, “A hallmark of Biden’s 2020 campaign is him losing it on voters and reporters when pressed about Ukraine.”

For his part, Mr. Biden has plenty of critical things to say about Mr. Trump, but he has publicly set a boundary for how he talks about the president. Campaigning in New Hampshire late last month, Mr. Biden noted that he had not “said a thing, and I’m not going to, about his family.”

“The way I was raised,” Mr. Biden said, “you don’t go after somebody’s kids.”

Thomas Kaplan reported from Sparks, and Katie Glueck from Osage, Iowa.