Biden allies also acknowledge he at times struggled during the Democratic debates, stumbling over his words or failing to parry attacks, like when Senator Kamala Harris, now his running mate, eviscerated him in a primary-campaign debate for his past stance on busing children to racially integrate schools.

And Biden’s team has tried to coach him not to get too caught up in trying to fact-check Trump, according to a person familiar with the campaign’s thinking. They worry too much time spent debunking Trump will take away from Biden’s opportunity to present his vision for the country. They expect moderator Chris Wallace of Fox News to do the fact-checking so Biden doesn’t have to.

“Debates don’t reward folks who speak in complete sentences and long paragraphs,” Carney said. “They really reward one-liners and insults and zingers.”

Still, the former vice president has made clear he expects to best Trump.

“He doesn’t know how to debate the facts,” Biden told MSNBC in an interview that aired Saturday. “He’s not that smart. He doesn’t know that many facts. He doesn’t know much about foreign policy. He doesn’t know much about domestic policy. He doesn’t know much about the detail.”