The presumptive Democratic nominee is aiming to expand the party's 2024 battleground map and appeal to young Black voters, delivering a firm, 20-minute speech to about 10,000 supporters in a packed arena and pledging Americans "are not going back" to the "failed policies" of Trump.
"Now, the baton is in our hands," Harris said to loud applause. "We have a fight in front of us... And we are the underdogs in this race."
Harris's nascent presidential bid took off following Biden's July 21 exit from the race, with much of the party coalescing behind her and her campaign raising a staggering $200 million.
"The momentum in this race is shifting, and there are signs that Donald Trump is feeling it," she said.
The Republican nominee recently said he would forego political tradition and not debate Harris, and also unleashed a barrage of insults against his rival, calling her "crazy" and a "bum."
"Well Donald, I do hope you'll reconsider to meet me on the debate stage, because as the saying goes, if you've got something to say, say it to my face," Harris said, to roars of approval.
Harris also repeated her popular line about how, as a former prosecutor and California attorney general going up against predators and fraudsters, she knows "Donald Trump's type."
"In this campaign, I will proudly pit my record against his any day of the week," she said.
- 'Extreme abortion bans' -
With just 98 days before the election, Harris is under pressure to announce her vice presidential pick. Asked Tuesday whether she had chosen one, Harris told reporters: "Not yet."
The search must be nearing a conclusion, however, as her team announced Tuesday that Harris and her new running mate would campaign next week in battlegrounds Pennsylvania, Wisconsin, Michigan, North Carolina, Georgia, Arizona and Nevada.
With the White House race turned on its head, 59-year-old Harris on Tuesday unveiled her first television ad since replacing Biden, while the Trump camp released a dueling spot attacking her on the crucial election issue of immigration.
Harris swatted away the attack, saying in her speech that while the Biden administration worked with conservatives to craft critical border legislation, Trump "tanked" it for political gain.
"Donald Trump does not care about border security," she said. "He only cares about himself."
Harris said if elected she would focus on key economic goals such as expanding affordable health care and tackling rising consumer costs.
She also attacked Trump, 78, over his "extreme abortion bans," referring to restrictive new laws enacted in several states in the two years since the US Supreme Court -- featuring three justices nominated by Trump -- stripped constitutional protections for abortion.
- Georgia in play -
As for Georgia's potential competitiveness, Harris insisted "the path to the White House runs right through this state."
In a sign the southern state will be bitterly contested, Trump and his running mate J.D. Vance announced they would hold their own rally Saturday in Atlanta.
"Kamala Harris and her complicit cronies have made the great people of Georgia pay a hefty price for their woke policies," the Trump campaign said Tuesday.
Harris took over a bleak electoral map from the faltering Biden, with Democrats' hopes entirely based on the three Rust Belt post-industrial states of Michigan, Wisconsin and Pennsylvania.
But they are now looking again at other "sunbelt" states such as Georgia, Arizona and Nevada, all of which Democrats narrowly won in 2020.
Vance addressed a rally Tuesday in Henderson, Nevada, where he attacked Harris as "dangerously liberal," while Trump campaigns in Pennsylvania on Wednesday.
Adding some Atlanta glitz for younger voters was Megan Thee Stallion, the hip-hop star who performed before Harris took the stage.
