Tim Walz has described the difficulty he and his wife Gwen had in conceiving a family, using the anecdote to fill the audience in with the story of his life.
Mr Walz, who isn’t widely known to Americans, also recalled growing up in a small town where he and his neighbours learned "to take care of each other."
"Everybody belongs and everybody has the responsibility to contribute," he says.
"But not everybody has that sense of responsibility," he goes on. "Take Donald Trump and J.D. Vance."
He runs through what Trump and Vance would do in power, including banning abortion.
"It's an agenda that nobody asked for. That serves nobody but the richest and most extreme amongst us and does nothing for our neighbours in need.
"Is it weird? Absolutely," he says to laughter. "But it's also wrong."
"I don't know about you, I'm ready to turn the page on these guys," he says, leading the crowd in the chant: "We're not going back."
"We've got something better to offer people," he says. "It starts with Kamala Harris. She has fought on the side of the American people," he adds.
"We owe it to the American people to tell them exactly what she would do as president," he adds, telling the crowd Ms Harris would cut taxes, take on Big Pharma and make home buying more affordable.
"No matter who you are she will fight for the freedom to stand up and live as you would like," he says.
Turning to the sporting phraseology of his days as a football coach, he tells the crowd: "It's the fourth quarter, we've got the field ball .. and boy do we have the right team. Kamala Harris is tough, Kamala Harris is ready. Our job is to get in the trenches and do the blocking and tackling.
"One phone call at a time, one door knock at a time, one $5 donation at a time. We've got 76 days. That's nothing.
"There will be time to sleep when you're dead. That's how we'll turn the page on Donald Trump," he says, his voice appearing to crack.
"As the next president of the United States says, 'When we fight'" he says, the crowd shouting back: "We win."
