"Let us fight for the ideals we hold dear and let us always remember, when we fight we win!" Harris, the US vice president, said in brief remarks that drew roars from the crowd.
She had been expected to appear later with President Joe Biden, the keynote speaker on Monday, the first night of a four-day event in Chicago.
Biden's appearance at the start of the four-day event caps a dramatic handover to his number two after he was pressured to quit the race last month by party leaders worried the 81-year-old incumbent was too old to win or serve another four years.
Having served as vice president to the first Black US president, Barack Obama, Biden bowed out of the race to allow his own second-in-command to try to make history as the first woman, a Black and Asian American, to hold the nation's highest office.
Joe Biden is expected to portray Trump as a threat at the Democratic National Convention. (AP PHOTO)
Harris is riding a historic whirlwind into the convention: her campaign has broken records for fundraising, packed arenas with supporters, and turned opinion polls in some battleground states in Democrats' favour.
Earlier she proposed to increase the corporate tax rate to 28 per cent from 21 per cent if she wins the November election.
Harris campaign spokesman James Singer said the move would be part of "a fiscally responsible way to put money back in the pockets of working people and ensure billionaires and big corporations pay their fair share".
When Trump was president, he slashed the corporate tax rate to 21 per cent from 35 per cent and implemented other tax breaks that are set to expire next year.
Trump has vowed to make the cuts permanent.
Changes to the US tax code require approval by Congress.
Trump pledged on Monday to rescind a Biden administration rule limiting power plant pollution and reiterated a promise to block Nippon Steel's planned purchase of US Steel.
"I will stop Japan from buying United States Steel," Trump said. "They shouldn't be allowed to buy it."
Donald Trump says he would "issue rapid approvals for new energy infrastructure" as president. (EPA PHOTO)
In the most detailed economic remarks he has offered on the campaign trail so far, Trump also ratcheted up his pledge to radically limit foreign access to domestic markets, saying he would ensure the supply chain for essential goods would be 100 per cent American if he wins the November 5 election.
He did not elaborate on how he would accomplish that goal beyond advocating broad measures, like imposing tariffs on other nations.
US Steel said in a statement after Trump's remarks that it is "committed to the transaction with Nippon Steel, which is the best deal for our employees, shareholders, communities, and customers."
The former US president spoke to a crowd of supporters on a factory floor in the working-class city of York, Pennsylvania.
Biden in March said it is vital for US Steel "to remain an American steel company that is domestically owned and operated".
Trump also pledged to eliminate rules published by the Environmental Protection Agency in April, which limit air and water pollution from power plants and are designed to cut over one billion metric tonnes of greenhouse gas emissions by 2047. The electricity sector is responsible for nearly a quarter of US greenhouse gas pollution, according to the EPA.
"It's a disaster for our country," Trump said. "Instead of shutting down power plants, we will open dozens and dozens more, and they'll happen fast."
While Democrats gathered at United Centre, home to Chicago's basketball and hockey teams, hundreds of protesters assembled at a nearby park to pressure the party to drop military support for Israel's offensive in the Gaza Strip.
Biden is due to speak at 12.50pm AEST on Tuesday and is expected to portray Trump as a threat to democracy while touting the achievements of the Biden-Harris administration.
Harris will formally accept the nomination on Thursday night with a speech.
