Tim Walz formally accepted his nomination to be the Democratic Party's vice presidential candidate in a keynote address on Wednesday at the party's convention in Chicago.

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

The Minnesota governor, who was little known in America 15 days ago, acknowledged he had never given such a big speech but said that as a former high school football coach, "I have given a lot of pep talks."

The crowd responded with chants of "Coach, Coach, Coach!" Walz brought his plain-speaking style and small-town values to the national stage, joined by a litany of political luminaries and star entertainers on the third night of the Democratic National Convention.

Walz said America should be a place where children don't go hungry, healthcare and housing are human rights, "and the government stays the hell out of your bedroom," referring to Republican assaults on abortion rights and same-sex marriage.

He spoke of preserving the freedoms that Democrats say are under attack from Trump, who is making his third major-party run for the White House.

Walz said a second Trump White House would serve nobody except the wealthy and most extreme.

Kamala Harris will address the gathering on its final night on Thursday when she will accept her party's nomination for president. (Agencies)