Hillary Clinton has declared victory in the Democratic party nomination race after winning at least three primaries on Tuesday night.
In an email to fundraisers seen by the Reuters news agency, Mrs Clinton said her campaign had broken “one of the highest, hardest glass ceilings”.
She wrote: “Together we secured the Democratic nomination.
“For the first time ever, a woman will be a major party’s nominee to become President of the United States.”
Barack Obama has called Mrs Clinton to congratulate her.
The 68-year-old becomes the first woman nominated by a major US political party to be president.
Addressing supporters at a victory rally in Brooklyn, New York, she said: “Thanks to you we have reached a milestone. The first time in our nation’s history that a woman will be a major party’s nominee.
“So many of you feel like you’re out there on your own. Nobody has your back. Well I do… and as your president I will always have your back.”
Mrs Clinton paid tribute to rival Bernie Sanders, saying their debates on income inequality in the US were good for America and the Democratic Party.
And she singled out Donald Trump for criticism, claiming he was “temperamentally unfit to be president and commander-in-chief”.
She won primaries held in New Jersey, South Dakota and New Mexico, but lost out to Mr Sanders in the North Dakota caucus.
According to an AP count on Monday, Mrs Clinton had already secured the 2,383 delegates she needed to clinch the nomination before Tuesday’s primaries and caucuses in six US states.
But the former US Secretary of State had been at pains to play down the claim, telling supporters in a rally in California that: “We are at the brink of a historic, historic unprecedented moment.”
She has wasted no time in turning her attentions to the general election after her campaign announced she will make stops next week in Ohio and Pennsylvania – two states that will be pivotal in November.
She will now face the task of having to win over Sanders supporters and to unify the Democratic party behind her.
Mr Sanders has repeatedly pledged to maintain his campaign up to the Democratic convention in July, and said on Tuesday he was “really disappointed” that Mrs Clinton had been declared the presumptive nominee on the eve of Tuesday night’s primaries.
The Vermont Senator will be hoping to win the state of California – the largest prize on offer on the night with 475 delegates – as he tries to persuade party superdelegates to back his campaign.
But early figures showed Mrs Clinton was on course to win the state.
Superdelegates – senior party figures who can choose who they vote for at the convention – are free to switch that vote. The vast majority have pledged themselves to backing Mrs Clinton.
On the Republican side, presumptive presidential nominee Donald Trump won all five primaries in New Jersey, New Mexico, Montana, California and South Dakota.
Speaking a victory rally in New York, he took aim at Mrs Clinton and her husband Bill, claiming they “had turned the politics of personal enrichment into an art form for themselves”.