The former US vice-president Joe Biden has announced he will seek the presidency, becoming the most prominent name to enter the crowded field of Democratic candidates competing to take on Donald Trump in the 2020 election.

He made his announcement in a video posted on Twitter, declaring: “We are in the battle for the soul of this nation. If we give Donald Trump eight years in the White House, he will forever and fundamentally alter the character of this nation. And I cannot stand by and watch that happen.”

He continued: “The core values of this nation, our standing in the world, our very democracy – everything that has made America America – is at stake. That’s why today I’m announcing my candidacy for president of the United States.”

It is Biden’s third run for the presidency, after two unsuccessful attempts at earning the Democratic nomination in 1988 and 2008.

Bolstered by his legacy as Barack Obama’s vice-president, Biden was expected to be an early frontrunner in a diverse Democratic field that features several heavyweight contenders but no clear favourite. In a statement issued by a spokesperson Thursday, the former president offered his praise – if not an endorsement, yet.

“President Obama has long said that selecting Joe Biden as his running mate in 2008 was one of the best decisions he ever made,” the spokesperson said, adding that the pair “remain close”.

Biden, 76, joins a jam-packed and diverse crop of candidates including the senators Bernie Sanders of Vermont, Elizabeth Warren of Massachusetts, Kamala Harris of California, Kirsten Gillibrand of New York and Cory Booker of New Jersey, as well as the Hawaii congresswoman Tulsi Gabbard and Julián Castro, the former mayor of San Antonio and housing secretary under Obama.

Donald Trump posted a threatening greeting to Biden on Twitter, saying: “Welcome to the race Sleepy Joe. I only hope you have the intelligence, long in doubt, to wage a successful primary campaign.”

If successful, Biden would become the oldest person to be elected president in US history. But allegations from several women that they were left feeling uncomfortable by their physical interactions with Biden have created some uncertainty over his prospects.

While Biden has not been accused of sexual assault or harassment, a mounting list of women have come forward to complain that he violated their personal space. The allegations prompted renewed scrutiny of what had long been dismissed as Biden’s affectionate posture and raised fresh questions over his viability as a candidate in the #MeToo era.

The controversy started with Lucy Flores, a former Nevada state assemblywoman who claimed in an op-ed that Biden had placed his hands on her shoulders, leaned in to smell her hair and kissed the back of her forehead at a political event in 2014. Other women subsequently came forward with similar stories, prompting an apology from Biden, who pledged in a video to adjust his behavior.

“Social norms are changing. I understand that, and I’ve heard what these women are saying,” Biden said. “Politics to me has always been about making connections, but I will be more mindful of personal space in the future. That’s my responsibility and I will meet it.”

Days later, Biden made light of the complaints in a speech where he twice jokingly referred to getting “permission” to hug people onstage.

As the Democratic party lurches left, emboldened by an animated progressive base, Biden is poised to serve as a more establishment-friendly voice whose legacy as vice-president was forged in part by leading negotiations with Republicans in Congress.

Following a recent revelation that he had praised a vulnerable House Republican in a paid speech weeks before the 2018 midterms, Biden offered an unapologetic response, stating: “I read in the New York Times today, one of my problems if I run for president is I like Republicans. Well, bless me father for I have sinned.”

Biden’s supporters say his appeal among blue-collar voters is key to defeating Trump and winning back what was once a core constituency of Democrats. Biden’s home state of Pennsylvania was crucial in sending Trump to the White House by a razor-thin margin over Hillary Clinton in the 2016 presidential election.

Biden, who publicly weighed running against Clinton in the 2016 primaries but ultimately chose not to, has suggested he could have defeated Trump. “I regret it every day,” Biden said of his decision.

But whether Biden’s record could withstand a Democratic primary in today’s political climate remains to be seen.

On one hand, Biden was lauded as vice-president for championing LGBTQ rights – endorsing same-sex marriage before Obama – and leading the White House’s charge for stricter gun laws and expansive labour rights. He also aggressively pushed for reforms to reduce sexual assault on college campuses, and as a senator drafted the landmark Violence Against Women Act.

At the same time, Biden has drawn scrutiny over his prior support for tough-on-crime legislation, which helped pave the way for mass incarceration.

He also faced widespread criticism in recent years for his tenure as the chair of the Senate judiciary committee in the early 90s, during which time the then supreme court justice nominee Clarence Thomas was accused by his former employee Anita Hill of sexual harassment. Biden has since apologized for his handling of the congressional hearings that followed, in which an all-male, all-white panel questioned Hill.

Before launching his campaign, Biden called Hill to personally apologize and express his regret over what she had “endured” during the hearings. But Hill told the New York Times that she left the conversation deeply unsatisfied.

“The focus on apology to me is one thing,” she told the paper. “But he needs to give an apology to the other women and to the American public because we know now how deeply disappointed Americans around the country were about what they saw. And not just women. There are women and men now who have just really lost confidence in our government to respond to the problem of gender violence.”

Since leaving the office of the vice-presidency, Biden has been a constant critic of Trump and accused the president of “trashing American values”.