Kamala Harris gains early momentum as she sharpens attack lines against Donald Trump

US Vice President Kamala Harris gained early momentum against Donald Trump on Tuesday as the Democrats' presumptive nominee sharpened attack lines against her Republican rival, a convicted felon.

Just two days after President Joe Biden dropped out of the race, Ms Harris opened a two-percentage-point lead over Mr Trump in a Reuters/Ipsos poll conducted on Monday and Tuesday.

Her advantage of 44 per cent to 42 per cent over Mr Trump is within the poll's margin of error, but it nonetheless marks a notable turnaround in recent polls, most of which have shown Mr Trump on course to easily prevail in November.

Democrats have reacted enthusiastically to Ms Harris's sudden emergence as presidential candidate, pouring hundreds of millions of dollars into her campaign and political action groups that support it.

In a preview of how she will take on Mr Trump, Ms Harris, who was California's attorney general from 2011-2017, is highlighting the former president's legal woes.

“I was a courtroom prosecutor. In those roles, I took on perpetrators of all kinds," she said on Monday to enthusiastic applause.

"Predators who abused women. Fraudsters who ripped off consumers. Cheaters who broke the rules for their own gain. So hear me when I say I know Donald Trump's type."

She repeated the lines at a rally in Wisconsin on Tuesday, to loud chants of "Kamala, Kamala".

Ms Harris also criticised Republicans for pushing Project 2025, a far-right proposal to reshape the federal government, and said Mr Trump would further erode America's frayed social safety net.

"We are not going back," she said.

Mr Trump has been found liable for sexual abuse and fraud, and was convicted of 34 felony counts of falsified business records. He has pleaded not guilty in other criminal cases against him.

Ms Harris, 59, quickly emerged as the Democratic candidate after Mr Biden, 81, announced on Sunday he would no longer seek a second term.

It came after weeks of pressure and grim polling, which showed Mr Biden had almost no path to victory after concerns about this age were highlighted during a debate against Mr Trump last month.

Within 36 hours of Mr Biden's endorsement of his deputy, she secured her party's nomination on Monday night by winning pledges from a majority of the delegates who will vote in the Democratic National Convention next month.

“I am proud to have secured the broad support needed to become our party's nominee,” Ms Harris said.

“I look forward to formally accepting the nomination soon.”

Ms Harris's rise dramatically reshapes an election in which many voters were unhappy with their options and offers Democrats a chance to reset their prospects after Mr Trump outperformed Mr Biden in poll after poll.

Saddled with concerns about his health and persistent high prices for goods ranging from petrol to housing, Mr Biden was behind in the competitive swing states that are likely to decide the election.

They include the Rust Belt states of Wisconsin, Michigan and Pennsylvania, and the Sun Belt states of Arizona and Nevada.

Mr Biden withdrew from the race after weeks of mounting pressure from Democrats who feared he lacked the stamina to take on Mr Trump.

The President disappeared from public view on July 17 after testing positive for Covid-19.

On Tuesday, he announced he would deliver a speech on Wednesday night from the Oval Office explaining his decision to end his campaign.

His exit followed Mr Trump's narrow survival of an assassination attempt that raised questions about security failures in the US Secret Service.

The agency director, Kimberly Cheatle, resigned on Tuesday, after numerous politicians called for her to step down.

Ms Harris's ascendancy to become the presumptive nominee has upended Mr Trump's campaign strategy.

After spending months attacking Mr Biden's physical and mental state, the 78-year-old former president is now the oldest person ever to seek the presidency and will face more scrutiny about his own health.

So far, Mr Trump and his allies have moved quickly to try to blame Mr Harris for some of Biden's more unpopular policies, including his administration's handling of the surge of migrants at the southern border with Mexico.

“Kamala Harris's dismal record is one of complete failure and utter incompetence. Her policies are Biden's policies, and vice versa,” Trump spokesman Steven Cheung said.