WASHINGTON (AFP) - For years Kamala Harris faced criticism that she was not up to the job of being a heartbeat away from the US presidency. Now, she finds herself feted by Democrats as their best hope to stop Donald Trump's comeback.

Despite blazing a trail as the first woman, Black and South Asian vice president in the nation's history, the 59-year-old Democrat long struggled with approval ratings as bad or worse than President Joe Biden's.

The last 12 months, however, have revealed a transformed Harris.

And with Biden's endorsement in hand, after he stunned the world by dropping his own reelection bid Sunday, she's suddenly on the cusp of history.

In a statement full of plaudits for Biden's achievements in office -- it was "unmatched in modern American history," she said -- Harris vowed to "earn and win" the nomination.

"I will do everything in my power to unite the Democratic Party -- and unite our nation -- to defeat Donald Trump," she said.

Harris will hope she has done the hard work to earn her full party's backing in the midst of the political crisis.

As the aging Biden seemed to visibly fade over the last year, his "veep" emerged as a force on the campaign trail, pushing for abortion rights and reaching out to core voters, including suburban women and Black men.

With a fondness for the f-bomb and her family nickname of "Momala" going viral, she has also finally started to cut through the noise to voters who previously paid scant attention.

Harris has won praise in party circles by staying loyal to the 81-year-old president even as political vultures began circling his candidacy.

She now appears likely to face Trump -- a brutal battle against a candidate who defeated Hillary Clinton in her bid to become the first female president in 2016.

- 'Ready to serve' - ==================== A child of immigrant parents -- her father was from Jamaica and her mother from India -- Harris grew up in Oakland, California, in an activist household that saw her attend her first rallies in a stroller.

Her focus on rights and justice saw her build an impressive CV, becoming California's first Black attorney general and the first woman of South Asian heritage elected to the US Senate.

Harris then went up against Biden in the 2020 primaries. In one stinging attack, she criticized him for allegedly opposing the busing of students to segregated schools.

"There was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day. And that little girl was me," she said in a barbed attack on her future boss.

But as his running mate, she consolidated the coalition that helped defeat the incumbent Trump in 2020. Her transition to the White House, however, proved difficult.

Critics said she was underwhelming and gaffe-prone in a job that has been known to flummox many officeholders.

Struggling to carve out a role, she was tasked by Biden with getting to the roots of the country's illegal migration problem, but fumbled and then got defensive in response to a question during a visit to the Mexican border.

Unusually high staff turnover fed rumors of discontent in the vice presidential office.

And Republicans -- often resorting to stereotypes her supporters branded as sexist and racist -- relentlessly targeted her as being unfit to take over, should the worst happen to America's oldest-ever president.

Harris told the Wall Street Journal in February: "I am ready to serve. There's no question about that."

- 'Momala' - ============ Things began to change as the 2024 race got underway. The Biden campaign repeatedly deployed Harris to battleground states to hammer home the party's message on abortion rights; she became the first vice president to visit an abortion clinic. Gradually, she began to draw more engaged and fired-up crowds.

Some of the outreach was, however, cringe-inducing. Earlier this year, she was mocked after she told chat show host Drew Barrymore that her family sometimes called her "Momala," and Barrymore replied, "We need you to be Momala of the country."

But voters seemed to be switching on.

A clip of her quoting her mother as often saying, "You think you just fell out of a coconut tree?" became a meme, with a rising sense among supporters that now could be her time.

If elected, Harris would break one of the highest glass ceilings left for women in the United States -- that of occupying the country's top office.

Her husband, Douglas Emhoff, would also be breaking new ground, moving from being the current Second Gentleman to the country's first First Gentleman.