Biden announced Sunday that he was stepping aside after a disastrous debate performance catalyzed fears that the 81-year-old was too frail for a second term.

Harris is the first woman, Black person or person of South Asian descent to serve as vice president. If she becomes the Democratic nominee and defeats Republican candidate Donald Trump in November, she would be the first woman to serve as president.

Biden said Sunday that choosing Harris as his running mate was "the best decision I've made" and endorsed her as his successor.

"Democrats -- it's time to come together and beat Trump," he wrote on X, the social media platform formerly known as Twitter. "Let's do this."

Harris described Biden's decision to step aside as a "selfless and patriotic act," saying he was "putting the American people and our country above everything else."

"I am honored to have the President's endorsement and my intention is to earn and win this nomination," Harris said. "Over the past year, I have traveled across the country, talking with Americans about the clear choice in this momentous election."

Prominent Democrats followed Biden's lead by swiftly coalescing around Harris on Sunday. However, her nomination is not a foregone conclusion, and there have been suggestions that the party should hold a lightning-fast "mini primary" to consider other candidates before its convention in Chicago next month.

A recent poll from the AP-NORC Center for Public Affairs Research found that about 6 in 10 Democrats believe Harris would do a good job in the top slot. About 2 in 10 Democrats don't believe she would, and another 2 in 10 say they don't know enough to say.

The poll showed that about 4 in 10 U.S. adults have a favorable opinion of Harris, whose name is pronounced "COMM-a-la," while about half have an unfavorable opinion.

A former prosecutor and U.S. senator from California, Harris' own bid for the 2020 Democratic presidential nomination imploded before a single primary vote was cast. She later became Biden's running mate, but she struggled to find her footing after taking office as vice president. Assigned to work on issues involving migration from Central America, she was repeatedly blamed by Republicans for problems with illegal border crossings.

However, Harris found more prominence as the White House's most outspoken advocate for abortion rights after the U.S. Supreme Court overturned Roe v. Wade in 2022. She has also played a key role in reaching out to young people and voters of color.

In addition, Harris' steady performance after Biden's debate debacle solidified her standing among Democrats in recent weeks.

Even before Biden's endorsement, Harris was widely viewed as the favorite to replace him on the ticket. With her foreign policy experience and national name recognition, she has a head start over potential challengers, including California Gov. Gavin Newsom, Michigan Gov. Gretchen Whitmer and Pennsylvania Gov. Josh Shapiro.

Harris will seek to avoid the fate of Hubert Humphrey, who as vice president won the Democratic nomination in 1968 after President Lyndon Johnson declined to run for reelection amid national dissatisfaction over the Vietnam War. Humphrey lost that year to Republican Richard Nixon.

Nixon resigned in 1974 during the Watergate scandal and was replaced by Vice President Gerald Ford. Ford never won a term of his own.

Vice presidents are always in line to step into the top job if the president dies or is incapacitated. However, Harris has faced an unusual level of scrutiny because of Biden's age. He was the oldest president in history, taking office at 78 and announcing his reelection bid at 80. Harris is 59.

She addressed the question of succession in an interview with The Associated Press during a trip to Jakarta in September 2023.

"Joe Biden is going to be fine, so that is not going to come to fruition," she stated. "But let us also understand that every vice president -- every vice president -- understands that when they take the oath they must be very clear about the responsibility they may have to take over the job of being president."

"I'm no different."

Harris was born Oct. 20, 1964, in Oakland, California, to parents who met as civil rights activists. Her hometown and nearby Berkeley were at the heart of the racial and social justice movements of the time, and Harris was both a product and a beneficiary.

She spoke often about attending demonstrations in a stroller and growing up around adults "who spent full time marching and shouting about this thing called justice." In first grade, she was bused to school as part of the second class to integrate Berkeley public education.

Harris' parents divorced when she was young, and she was raised by her mother alongside her younger sister, Maya. She attended Howard University, a historically Black school in Washington, and joined the Alpha Kappa Alpha sorority, which became a source of sisterhood and political support over the years.

After graduating, Harris returned to the San Francisco Bay Area for law school and chose a career as a prosecutor, a move that surprised her activist family.

She said she believed that working for change inside the system was just as important as agitating from outside. By 2003, she was running for her first political office, taking on the longtime San Francisco district attorney.

Few city residents knew her name, and Harris set up an ironing board as a table outside grocery stores to meet people. She won and quickly showed a willingness to chart her own path. Months into her tenure, Harris declined to seek the death penalty for the killer of a young police officer slain in the line of duty, fraying her relationship with city cops.

The episode did not stop her political ascent. In late 2007, while still serving as district attorney, she was knocking on doors in Iowa for then-candidate Barack Obama. After he became president, Obama endorsed her in her 2010 race for California attorney general.

Once elected to statewide office, she pledged to uphold the death penalty despite her moral opposition to it. She refused to defend Proposition 8, a voter-backed initiative banning same-sex marriage. Harris also played a key role in a $25 billion settlement with the nation's mortgage lenders following the foreclosure crisis.

As killings of young Black men by police received more attention, Harris implemented some changes, including tracking racial data in police stops, but didn't pursue more aggressive measures such as requiring independent prosecutors to investigate police shootings.

Harris' record as a prosecutor would eventually dog her when she launched a presidential bid in 2019, as some progressives and younger voters demanded swifter change. But during her time on the job, she also forged a fortuitous relationship with Beau Biden, Joe Biden's son who was then Delaware's attorney general. Beau Biden died of brain cancer in 2015, and his friendship with Harris figured heavily years later as his father chose Harris to be his running mate.

Harris married entertainment lawyer Douglas Emhoff in 2014, and she became stepmother to Emhoff's two children, Ella and Cole, who referred to her as "Momala."

Harris had a rare opportunity to advance politically when Sen. Barbara Boxer, who had served more than two decades, announced she would not run again in 2016.

In office, Harris quickly became part of the Democratic resistance to Trump and gained recognition for her pointed questioning of his nominees. In one memorable moment, she pressed now-Supreme Court Justice Brett Kavanaugh on whether he knew any laws that gave government the power to regulate a man's body. He did not, and the line of questioning galvanized women and abortion rights activists.

A little more than two years after becoming a senator, Harris announced her campaign for the 2020 Democratic presidential nomination. But her campaign was marred by infighting and she failed to gain traction, ultimately dropping out before the Iowa caucuses.

Eight months later, Biden selected Harris as his running mate. As he introduced her to the nation, Biden reflected on what her nomination meant for "little Black and brown girls who so often feel overlooked and undervalued in their communities."

"Today, just maybe, they're seeing themselves for the first time in a new way, as the stuff of presidents and vice presidents," he said.