Tim Walz made his mark at the third night of the Democratic National Convention (DNC) as he formally accepted his nomination as Kamala Harris’s White House running mate on Wednesday.

"It's the honour of my life to accept your nomination for vice president of the United States. What all we're all here tonight for one beautiful, simple reason. We love this country," Walz told the DNC in Chicago.

During his energetic fifteen-minute speech, Walz shared his humble beginnings in small-town Nebraska and his journey from working on a family farm to becoming a teacher, where he found inspiration for his political career, AFP reported.

"They saw in me what I had hoped to instill in them: a commitment to the common good, an understanding that we're all in this together, and the belief that a single person can make a real difference for their neighbours," he said as the crowd chanted "Coach Walz!"

The 60-year-old Minnesota governor didn't hold back in criticising Republican nominee Donald Trump, accusing him of divisiveness and blame-shifting. However, the primary focus of his address at the United Center in Chicago was to champion Harris and rally support for her.

"From her day as a prosecutor, as a district attorney, as an attorney general, as a United States senator and then our vice president, she's fought on the side of the American people," he said.

"She's taken on the predators and fraudsters. She's taken down the transnational gangs... She has never hesitated to reach across that aisle if it meant improving your lives, and she's always done it with energy, with passion and with joy."

Introduced by Minnesota first lady Gwen Walz, the former National Guard soldier was cheered boisterously as he took to the stage for the biggest speech of his political career, ahead of Harris's big night Thursday.

In a moment that brought the audience to its feet, former students marched onstage to introduce Walz — a football coach in his teaching days — after a musical interlude by Grammy Award-winning musician John Legend.

Walz's main job was to tee up Harris's speech and the climax of a convention that has defied predictions of disunity or even chaos in the wake of President Joe Biden's decision to drop his reelection bid on July 21.

"No matter who you are, Kamala Harris is going to stand up and fight for your freedom to live the life that you want to lead, because that's what we want for ourselves and it's what we want for our neighbours," he said.

He has made a name for himself as an able communicator and is credited with coming up with one of the sharpest attack lines on Trump and his running mate JD Vance, whom he labeled "weird."

The chemistry between 59-year-old Harris and Walz and the noisy energy generated at their rallies is fueling Democratic hopes that they can defeat Trump, 78, in November.

Polls show the election remains close, but Harris is moving slightly ahead, a remarkable turn of events, given that only a month ago Trump seemed to be gaining a steadily tightening grip on the race.

On the convention floor, delegates praised Walz's keynote address.

"I thought his speech was perfect," New York delegate Edwina Martin, 60, told AFP.

"I heard him at rallies say a lot of the same things, but tonight, hearing it in this setting was incredibly powerful and meaningful and inspiring."

Before Walz lit up the arena, the crowd heard from former president Bill Clinton.

"In 2024, we have a clear choice: 'We The People' versus 'Me, Myself, and I,'" Clinton said.