WASHINGTON: In a major development, US Vice President Kamala Harris on Monday won the crucial backing of Democratic heavyweight Nancy Pelosi to lead the party against Donald Trump in November after Joe Biden's stunning exit from the 2024 race.

As the endorsements stacked up, the 59-year-old Harris made her first public appearance since Biden's announcement in a ceremony at the White House where she warmly praised the outgoing president's "unmatched" achievements.

But while she steered away from any triumphalism, Harris will now feel she has one hand on the prize after securing former US House speaker Pelosi's support.

"With immense pride and limitless optimism for our country's future, I endorse Vice President Kamala Harris for President of the United States," Pelosi, 84, said on X.

"I have full confidence that she will lead us to victory in November."

A flood of Democratic leaders have backed Harris as the party's new candidate, building momentum for a lightning-fast coronation despite some calls to show transparency with an open primary.

Biden, 81, endorsed Harris -- who is the first female, Black and South Asian vice president in US history -- as he dropped out of the race on Sunday in the culmination of a crisis triggered by a disastrous performance at a debate with Trump.

Then came endorsements from former president Bill Clinton and a host of lawmakers, but notably ex-president Barack Obama has held off so far.

'Unmatched'

In a strikingly symbolic moment, Harris hosted a ceremony for college athletes at the White House on Monday while Biden remained stuck in isolation with Covid at his Delaware beach house.

"Joe Biden's legacy of accomplishment over the past three years is unmatched in modern history," Harris said in her brief remarks on the White House South Lawn, as a light rain fell.

Some of her sporting metaphors did seem to nod towards the political race ahead of her, though, as she talked of bringing home the gold and "what it means to commit and to persevere."

Harris will make a first trip to campaign headquarters in Wilmington, Delaware, later Monday -- not far from Rehoboth Beach, where Biden has been nursing his Covid infection since last week.

Biden's symptoms "have almost resolved completely," his doctor said in a statement Monday, though the White House has not yet announced any events on his schedule this week.

Harris's campaign said it had raised a stunning $49.6 million in grassroots donations since Sunday.

A series of other top Democrats have backed Harris, including a number considered as her possible running mates.

"Let's win this," posted Michigan Governor Gretchen Whitmer.

The governor of Kentucky, Andy Beshear, also declared his support, as did Illinois Governor JB Pritzker.

Harris must still win over some key hold-outs if she is to wrap up the nomination, but it could happen as early as a remote ballot on August 1, or failing that by the Democratic National Convention starting August 19.

'Threat to democracy'

Biden's stunning withdrawal has completely upended the 2024 race, transforming a long slog between two unpopular elderly men into one of the most compelling in modern US presidential history.

The move has brought a jolt of energy to a demoralized party that Harris could now unify, and could give America its first female president.

It has also hit Republicans hard, with former president Trump, 78 - now the oldest presidential nominee in US history - having to completely retool a strategy that had been built around attacking Biden over his age and physical frailty.

Harris's entry not only flips the age issue but puts Trump -- a convicted felon who has faced a series of legal cases over sexual assault -- up against a woman and former prosecutor.

Trump has seemed to find it hard to move on from his old opponent.

He launched a series of invective-filled social media posts on Sunday and Monday after Biden quit, mocking the president's age and saying he and Harris posed a "threat to democracy."

The challenges facing Harris remain daunting, however, with less than four months until election day.

The vice president has long suffered from poor approval ratings after a lackluster first two years in the White House. She is polling largely neck-and-neck with Trump in the polls that have looked at a direct match up.