Harris, 59, was the sole candidate on the ballot for a five-day electronic vote of nearly 4,000 party convention delegates. The first Black and South Asian woman ever to secure a major party's nomination, she will be officially crowned at a Chicago convention later this month.

Harris said on a phone-in to a party celebration she was "honored" to have amassed the required support by the second day of the marathon virtual vote and declared: "We are going to win this election." 

"And it is going to take all of us... We are going to talk with people about the fact that we are all in this together, and we stand together," Harris said.

"And so, let's let folks know that our campaign is about the future. And it's about an expansion of rights and freedoms, and for the opportunity of everyone to not just get by, but to get ahead."

In the two weeks since Joe Biden ended his reelection bid, Harris has gained full control of the party, smashing fundraising records, packing arenas and erasing the polling leads Trump had built over the president.

"I couldn't be prouder," Biden posted on X after her nomination.

The nomination milestone came with Harris preparing to hit the campaign trail next week for a swing across seven crucial election states alongside her yet-to-be-named running mate.

The roll call -- held earlier than usual and online due to altered state registration rules -- marks the official beginning of the 2024 convention, with the traditional festivities starting when the party faithful descends on Chicago on August 19.

Wave of momentum -

Trump's White House bid was turned upside down on July 21 when 81-year-old Biden, facing growing concerns about his age and lagging polling numbers, withdrew his candidacy and backed Harris.

Energetic and two decades younger than 78-year-old Trump, the vice president has made a fast start, raising $310 million in July, according to her campaign -- more than double Trump's haul. 

She and her running mate are scheduled to rally Tuesday in Pennsylvania -- a crucial swing state, where Democratic Governor Josh Shapiro is on the shortlist to join Harris's ticket.

Biden beat Trump in Pennsylvania in 2020 by around 80,000 votes and it is seen as the biggest prize of the closely fought battlegrounds that decide the Electoral College system.

The Keystone State is part of the so-called blue wall that carried Biden to victory in 2020, alongside Michigan and Wisconsin, two states where Harris is due to woo crowds on Wednesday.

She will also tour the more racially diverse Sun Belt and southern states of Arizona, Nevada, Georgia and North Carolina as she seeks to shore up the Black and Hispanic vote that had been peeling away from the Democrats. 

In a sign that the Harris campaign is thinking big, US media reported that a raft of senior advisors from Barack Obama's historic candidacies in 2008 and 2012 have taken up top positions with her.

While Biden made high-minded appeals for a return to civility and the preservation of democracy, Harris has focused on the future, making voters' hard-fought "freedom" the touchstone of her campaign.

She and her allies have also been more aggressive than the Biden camp -- mocking Trump for reneging on his commitment to a September debate and characterizing the convicted felon as an elderly crook and "weird." 

Meanwhile Trump and his Republicans have struggled to adapt to their new adversary or hone their attacks against Harris -- at first messaging that she was dangerously liberal on immigration and crime before pivoting to falsely accusing her of pretending to be Black for political purposes.