Joe Biden profile: Third White House run for 'Middle Class Joe'
Former US Vice-President Joe Biden is staging his third run for the Democratic presidential nomination.
This time, he entered the race as the de facto frontrunner, the most well-known and well-liked candidate in the field.
The 77-year-old suffered early setbacks in the Iowa caucuses and New Hampshire primary but seized control of the contest with a string of successes on Super Tuesday.
Although Mr Obama called him the "best vice-president America has ever had", Mr Biden's record from four decades in public office has come under attack. So who is he, and what's behind his bid?
A political fixture
Mr Biden ran for the Democratic 2008 nomination before dropping out and joining the Obama ticket.
His eight years in the Obama White House - where he frequently appeared at the president's side - has allowed Mr Biden to lay claim to much of Mr Obama's legacy, including passage of the Affordable Care Act, the economic stimulus package and financial industry reform.
His oft-touted association with Mr Obama - a man he frequently calls his "brother" - may also contribute to his enduring support among African American voters.
As a long-time Washington insider, Mr Biden had solid foreign affairs credentials, and helped balance Mr Obama's comparative lack of political experience.
The so-called "Middle Class Joe" was also brought on board to help woo the blue-collar white voters who had proved a difficult group for Mr Obama to win over.
He made headlines in 2012 by saying he was "absolutely comfortable" with same-sex marriage, comments that were seen to undercut the president, who had yet to give full-throated support for same-sex marriages. Mr Obama ultimately did so, just days after Mr Biden.
These two terms supporting the first black president marked a high point of a nearly 40-year political career for Mr Biden.
The six-term senator from Delaware was first elected in 1972. He first ran for president in 1988 but withdrew after he admitted to plagiarising a speech by the then leader of the British Labour Party, Neil Kinnock.
His lengthy tenure in the nation's capital has given critics ample material for attacks.
Early in his career, he sided with southern segregationists in opposing court-ordered school busing to racially integrate public schools.
And, as chair of the Senate Judiciary Committee in 1991, he oversaw Clarence Thomas's Supreme Court confirmation hearings and has been sharply criticised for his handling of Anita Hill's allegations that she was sexually harassed by the nominee.
Mr Biden was also a fierce advocate of a 1994 anti-crime bill that many on the left now say encouraged lengthy sentences and mass incarceration.
The record has made Mr Obama's moderate vice-president a sometimes uncomfortable fit for the modern Democratic Party.
Mr Biden's life has been stained by personal tragedy.
In 1972, shortly after he won his first Senate race, he lost his first wife, Neilia, and baby daughter, Naomi, in a car accident. He famously took the oath of office for his first Senate term from the hospital room of his toddler sons Beau and Hunter, who both survived the accident.
In 2015, Beau died of brain cancer at the age of 46. The younger Biden was seen as a rising star of US politics and had intended to run for Delaware state governor in 2016.
The elder Biden has credited Beau with encouraging him to run again, and has used both tragedies on the trail to explain why healthcare - one of his signature policy goals - is "personal" to him.
Mr Biden garnered considerable goodwill following Beau's death, which served to highlight Mr Biden's central strengths: a reputation as a kind and relatable family man.
This perceived warmth is not without its pitfalls. After entering the 2020 race, he fended off accusations of unwelcome physical contact during interactions with female voters - complete with uncomfortable accompanying footage.
But the avuncular politician responded by saying he is an empathetic person, though he accepted standards had changed. The episode, however, stoked a perception for some that he was out of touch.