Former vice president Joe Biden has officially entered the US election race, presenting Donald Trump with his most popular challenger yet.
The 76-year-old is the most recognised Democratic candidate yet to throw his hat into the ring, reports news.com.au.
He enjoys respect and affection from within the Democratic Party, and became something of an online phenomenon among the public during the Obama years.
There are reports Mr Trump has long feared this scenario, but Mr Biden faces his own batch of controversies which could prove problematic.
BIDEN SLAMS TRUMP IN CAMPAIGN ANNOUNCEMENT
In announcing his candidacy, Mr Biden wasted no time in hitting out at Mr Trump.
He posted a viral social media video that warned history will look back terribly on the celebrity billionaire's four years in office.
His first two words were "Charlottesville, Virginia" — a reference to the President's reaction to white supremacists marching in Charlottesville and killing a counter-protester, which was to praise the "very fine people on both sides".
"With those words, the President of the United States assigned a moral equivalence between those spreading hate and those with the courage to stand against it," Mr Biden said in his announcement video. "And in that moment, I knew the threat to this nation was unlike any I'd ever seen in my lifetime.
"We are in the battle for the soul of this nation," he continued. "I believe history will look back on the four years of this President, and all he embraces, as an aberrant moment in time."
Mr Biden's approach to the President was notably head-on compared to the other Democratic candidates, whose strategies have been to focus more on their own campaigns and less on Mr Trump.
Mr Trump responded to the video with a sarcastic tweet "welcoming" Mr Biden into the race:
His advisers say his flippant response is a sign the President is terrified.
TRUMP PRIVATELY FEARS BIDEN RUNNING
Mr Biden is undoubtedly popular — both in the Democratic party and among voters broadly.
Earlier this year, Democratic Senator Chris Murphy described him as "one of the most beloved figures in the party", while others noted he has a wealth of experience and is politically centred enough to appeal to a broad range of Democrats.
In the wider public, he boosted his profile working alongside former president Barack Obama, and has previously claimed he is "the most qualified person in the country to the president".
While it's early days, the polls support this. A recent Morning Consult-Politico poll, published 24 hours before Mr Biden announced his run, put him ahead of Mr Trump by eight points in a hypothetical 2020 match-up.
The poll gave Mr Biden a strong lead over Mr Trump on women, millennials and independents.
The same poll gave him a remarkably positive favourability ratio among Democrats, with a diverse base of support.
One could argue polls are insignificant at this stage. But they're not insignificant to Mr Trump, who repeatedly cites the numbers when he can use them to make a point.
At the same time, there are reports Mr Trump has been fearful of a Biden run for a long time.
According to Politico, the President just six months ago was talking privately with aides about the threat posed by the former vice president, saying: "How are we gonna beat Biden?"
When reassured Mr Biden would never defeat his rivals within the Democratic party, Mr Trump pushed back: "But what if he does?"
A Sky News analysis described Mr Biden as "the Democrat that President Trump most fears".
A Vanity Fair report went even further, saying the President is "having a full-blown meltdown over Joe Biden".
White House aides have previously told Axios in the past that he fears Mr Biden could flip Pennsylvania back into a Democratic state, and win a general election.
The source of his fear, Politico speculates, is that the two have so much in common. Both Mr Trump and Mr Biden are 70-something white men who take an unscripted, speak-off-the-cuff approach to politics.
Both tend to target and have a proven ability to win over less-educated blue-collar voters.
They take similar swipes at one another too. In the lead-up to the 2016 election, Mr Biden famously told a Pennsylvania rally: "The press always ask me, 'Don't I wish I were debating him?' No, I wish we were in high school — I could take him behind the gym. That's what I wish."
Mr Trump hit back: "You know when he said, 'I'm going to take you behind the barn'? Oh, just trust me, I would kick his ass. He'd be easy."
Even their controversies are similar — a trend that may present an immediate hurdle to Mr Biden's run.
Mr Trump may have a poor track record when it comes to women, but Mr Biden's — who in recent months has come under fire for inappropriate behaviour towards women — is no better.
His decision to portray himself as a champion of people of colour in his announcement video has also faced criticism.
This could put him at a disadvantage next to fellow Democratic candidates like California senator Kamala Harris, who has chosen women of colour to drive her political campaign.
Mr Trump has denied fearing Mr Biden. Last year, he told CBS News he "dreamed" of running against Mr Biden.
"I don't see Joe Biden as a threat. No, I don't see him as a threat. I think he is only a threat to himself," the President said. "He's been there a long time. His record's not good. He'd have to run on the Obama failed record."
DARK SIDE TO 'UNCLE JOE'
Mr Biden may be popular overall, but his reputation has increasingly been called into question over the past few months — namely due to a string of complaints from women he's worked with.
Last month, Democrat Lucy Flores penned a viral first-person essay in which she alleged he
kissed her on the back of the head moments before they were due to give a joint speech in Nevada back in 2014.
"I felt him get closer to me from behind. He leaned further in and inhaled my hair. I was mortified. I thought to myself, 'I didn't wash my hair today and the vice-president of the United States is smelling it. And also, what in the actual f**k? Why is the vice-president of the United States smelling my hair?'
"He proceeded to plant a big slow kiss on the back of my head. My brain couldn't process what was happening. I was embarrassed. I was shocked. I was confused. There is a Spanish saying, "tragame tierra," it means, "earth, swallow me whole." I couldn't move and I couldn't say anything."
Mr Biden denied the incident took place. But his history of conduct with women has been well-documented in the US press.
In 2013, an awkward photograph emerged of him embracing TV reporter Amie Parnes in front of a Christmas tree, with his hands positioned just below her breasts.
In the photo, her hands are firmly clasped over his, in what appears to be an attempt to keep them from moving any higher.
In a 2015 incident, he came under fire after he grabbed the arm of a pre-teen senator's daughter, whispered into her ear and kissed the side of her forehead.
She looked visibly uncomfortable with the exchange.
He's also touched women's faces and necks in various other photo ops.
Such incidents have earned him nicknames in some sectors of the American media, such as "Handsy Joe" and "Uncle Joe", but he was rarely criticised directly for them until now.
Mr Biden has also copped criticism for using Charlottesville in his campaign, with some arguing that he ignored the issue of structural racism prior to his announcement, and is now using it as a "prop".
Mr Biden may also face trouble with young voters if he proves himself to be out of touch, having previously had harsh words for millennials.
He made headlines last year after stating that he has "no empathy" for millennials.
"The younger generation now tells me how tough things are. Give me a break. No, no, I have no empathy for it. Give me a break," he told the LA Times, citing his own generation's achievements.