The online voting process does not end until Monday but the campaign marked the moment when she crossed the threshold to have the majority of delegates' votes.
Harris needed to pass a 2350-vote threshold from delegates to formally secure the nomination, her campaign said.
"I am honoured to be the presumptive Democratic nominee," Harris said on a call with supporters.
Harrison said "we will rally around Vice President Kamala Harris and demonstrate the strength of our party" during its convention in Chicago later this month.
Kamala Harris has secured the delegates to soon become the presidential nominee for the Democratic Party. — The Democrats (@TheDemocrats) pic.twitter.com/qcDvube5BuAugust 2, 2024
Democrats have pushed ahead with a virtual vote to nominate Harris, nearing the culmination of a turbulent process that was up-ended by President Joe Biden's decision not to seek re-election.
Delegates to the Democratic National Convention began voting via secure email on Thursday, and the voting will remain open until Monday evening.
Harris has not yet chosen her running mate and she is expected to interview candidates over the weekend.
The formal nomination is expected to be finalised by August 7 even though the party's convention in Chicago is not scheduled to begin for more than two more weeks.
Democratic officials have said the accelerated timeline was necessary because of an August 7 deadline to ensure candidates appear on the Ohio ballot.
Harris was endorsed by Biden shortly after he dropped out of the race, catapulting her to the forefront of the campaign to beat Republican nominee Donald Trump.
No other major candidate challenged Harris for the nomination and she was the only choice for delegates under party rules that required pledges of support from at least 300 delegates, with no more than 50 signatures from any one delegation.
Any delegate who wants to vote for someone other than Harris will be tallied as "present".
Democrats still plan a state-by-state roll call during the convention, the traditional way that a nominee is chosen.
However, that will be purely ceremonial because of the online voting.
The party insists it has to have its nominee in place before its convention opens in Chicago on August 19 to make sure it meets ballot access deadlines in Ohio - an argument that the state's Republicans dispute.
Ohio state lawmakers have since changed the deadline but the modification does not take effect until September 1.
Democratic lawyers warn that waiting until after the initial deadline to determine a presidential nominee could prompt a legal challenge.
Harris added two former aides to Barack Obama to her presidential campaign team, a source told Reuters on Friday.
David Plouffe, a US political strategist who ran Obama's successful 2008 presidential campaign, has joined Harris' campaign for president as a senior adviser, the source said.
The campaign is also being joined by Stephanie Cutter, who previously served as Obama's White House communications director and deputy campaign manager, the source said.
with Reuters
