Former US Vice-President Joe Biden has declared a presidential bid, putting an end to months of speculation.
In a video announcement, Mr Biden warned that the "core values of the nation...our very democracy, everything that has made America America, is at stake".
The 76-year-old enters a crowded race for the 2020 Democratic nomination.
He is up against 19 other hopefuls, including Senators Elizabeth Warren, Kamala Harris, and Bernie Sanders.
In his announcement, Mr Biden recalled President Donald Trump's much-criticised response to the deadly Charlottesville white nationalist riots of 2017, saying the US was in a "battle for the soul of this nation".
"I believe history will look back on four years of this president and all he embraces as an aberrant moment in time," he said.
"But if we give Donald Trump eight years in the White House, he will forever and fundamentally alter the character of this nation, who we are, and I cannot stand by and watch that happen."
Mr Biden is the most experienced of the many Democratic candidates. A six-term senator, he served as President Barack Obama's deputy for two terms and ran twice unsuccessfully for president - in 1988 and 2008.
He was tipped to run again in 2016, but ruled himself out after the death of his 46-year-old son Beau Biden from a brain tumour.
Since the vice-presidency, Mr Biden has enjoyed relative popularity among Democrats. On some progressive issues, such as same-sex marriage, he was ahead of Mr Obama.
His popularity is reflected in opinion polls - he has consistently led every national poll of the Democratic primary tracked by the website RealClearPolitics.
The sheer weight of his experience sets him apart from many of the younger 2020 Democratic hopefuls, and widespread national popularity and name recognition make him an immediate front-runner.
But Mr Biden also carries political baggage that the liberal wing of his party sees as problematic—including support for the Iraq war, opposition to efforts to improve racial integration, and controversy over his 1991 handling of sexual harassment allegations against Supreme Court nominee Clarence Thomas.
There is also the question of Mr Biden's age. On inauguration day he would be 78, making him the oldest sitting president in history, at a time when many Democratic voters are looking to a younger generation to galvanise the party.
Who is Joe Biden?
Joseph Robinette Biden Jr was born on 20 November 1942 in Scranton, Pennsylvania, one of four children in an Irish-Catholic family.
In 1972, Mr Biden was elected to the US Senate at the age of 29, and took office a few weeks later when he turned 30 - the minimum age to enter the Senate.
Just before he took office, he was devastated by tragedy: his wife Neilia and infant daughter Naomi were killed in a car crash.
Mr Biden first ran for the presidency in the 1988 election, but withdrew after admitting that he had plagiarised a speech by Neil Kinnock, the leader of the Labour Party in the UK at the time.
After that bid he spent time rising through the Senate ranks, eventually becoming chairman of the judiciary and foreign relations committees.
In 2008 he ran for president again, but failed to gain the political traction he needed and dropped out again. Instead, he joined the Obama ticket as candidate for vice-president.