Vice presidential nominee Tim Walz and former President Bill Clinton are headlining the Democratic National Convention on Wednesday, the third day of the party’s choreographed rollout of a new candidate, Kamala Harris, and her pitch to voters.

Mr Walz faces dual tasks in his speech in Chicago. The Minnesota governor, who has become known among supporters as a folksy, Midwestern teacher, coach and dad, will introduce himself to the rest of the country. He will also need to articulate the campaign’s central message, and theme for the evening: that a second Donald Trump term would imperil American liberties.

The third night of the convention marks a transition from Tuesday’s exuberant party atmosphere to a theme that organisers have dubbed “a fight for our freedoms”.

Ms Harris is portraying former president Mr Trump as a threat to democracy in general as well as to abortion rights and other personal choices in particular.

To help shepherd the campaign’s message, Mr Walz will be joined by Mr Clinton – a veteran of the political convention speech — as well as a mix of the party’s other top stars.

They include transportation secretary Pete Buttigieg, former House Speaker Nancy Pelosi and House minority leader Hakeem Jeffries.

Convention organisers say those speeches will be balanced by remarks from what they describe as “everyday Americans” whose freedoms hinge on the upcoming election.

Mini Timmaraju, president and chief executive of the non-profit Reproductive Freedom for All, is expected to speak on reproductive rights, which Ms Harris’s campaign sees as a key motivator for Democratic voters this fall.

Wednesday’s program also includes two governors, Wes Moore of Maryland and Josh Shapiro of Pennsylvania, and several Democratic senators: Cory Booker of New Jersey, Chris Murphy of Connecticut, Catherine Cortez Masto of Nevada and Amy Klobuchar of Minnesota.

Mr Trump, meanwhile, will hold his first outdoor rally since he survived an assassination attempt in Pennsylvania last month. He will give remarks at the North Carolina Aviation Museum & Hall of Fame, his podium surrounded by bulletproof glass.

Many Americans had never heard of Mr Walz until Ms Harris chose him to join her ticket. In his first weeks of campaigning, he has charmed supporters with his background as a teacher and football coach and helped to balance Ms Harris’ coastal background as a cultural representative of Midwestern states whose voters she needs this fall.

But Mr Walz also has faced scrutiny, including questions about embellishing his background. His wife this week clarified that she did not undergo in vitro fertilisation (IVF), as Mr Walz has repeatedly claimed, but used other fertility treatments.

Republicans also have criticised Mr Walz for a 2018 comment he made about carrying weapons in war. Though he served in the National Guard for 24 years, he did not deploy to a war zone.

Still, polling data shows that despite early hiccups, Mr Walz has had a smoother launch as Ms Harris’ running mate than Republican JD Vance, Mr Trump’s pick for vice president.

About one-third of US adults (36%) have a favourable view of Mr Walz, while about one-quarter (27%) have a positive opinion of Mr Vance, according to a new poll from The Associated Press-NORC Centre for Public Affairs Research. Significantly more adults also have an unfavourable view of Mr Vance than Mr Walz, at 44% to 25%.

The week so far has been a balancing act for Ms Harris and other Democrats to harness the exuberance that has swept over their party since President Joe Biden stepped aside, while making clear to their supporters that the election will be a fierce fight.

In Chicago, the convention’s second day toggled between light-hearted celebration and more serious warnings. A symbolic roll call vote turned into a dance party, featuring surprise musical acts like Lil Jon, who performed Turn Down for What, his song with DJ Snake.

But former President Barack Obama also conveyed a more sober message in his keynote speech, saying, “Make no mistake, it will be a fight”.

“So much is on the line in this election,” Ms Harris said Tuesday in Milwaukee, where she spoke at a professional basketball arena in battleground Wisconsin as the convention continued 90 miles away in Chicago. “And understand, this is not 2016 or 2020. The stakes are higher.”

Ms Harris will be back in Chicago on Wednesday for the convention’s third night.