Joe Biden on Monday accused US President Donald Trump of abusing the powers of his office and ignoring everyone but his political base.
In his first public appearance as presidential candidate, the former vice president under Barack Obama held a rally in the crucial battleground state of Pennsylvania.
Mr Biden said that strengthening unions and promoting social and economic unity could restore the Democrats to the White House.
He told hundreds of supporters and labour activists that Saturday's deadly shooting at a California synagogue proved again that the country is "in a fight for its soul".
But Mr Biden spent more time criticising Mr Trump for his political behaviour than for his past comments about white supremacists.
“There’s only one thing that stands in our way," he said. "It’s our broken political system that’s deliberately being undermined by our president to continue to abuse the power of the office."
Mr Biden said Mr Trump was “the only president who has decided not to represent the whole country. He has his base. We need a president who works for all Americans".
Earlier in the day, he received the endorsement of the International Association of Fire Fighters, and many in the crowd wore union T-shirts and carried signs supporting him.
They repeatedly interrupted him with cries of “We want Joe” and the candidate declared: “I make no apologies. I am a union man,” to sustained applause.
Mr Biden’s 40-minute speech swung from vows of union solidarity to broadsides against Mr Trump, but lacked the sharp focus on the president that marked his campaign announcement video last week.
Instead, Mr Biden returned to his familiar, folksy pitch to working-class voters and avoided the sweeping, liberal positions embraced by much of the rest of the crowded 2020 Democratic field.
Mr Trump, meanwhile, stepped up his criticism of the Democrat and boasted on Twitter about his own strong support among union members, before his rival gave his pro-union message in Pittsburgh.
He won Pennsylvania in surprising fashion in 2016, but Mr Biden promised that his party would improve its standing by bringing the country together, not further dividing it.
Mr Biden, 76, now leads national polls of Democratic preference for the party’s 2020 primary, and in Iowa, which votes first in the contest.
He raised more than any of his 20 rivals in the first 24 hours of their official candidacies. But his age could work against him as the party looks for new blood.
Mr Biden's video announcing his White House bid last week began with the words, “Charlottesville, Virginia”.
It was a new condemnation of what many feel is a stain on Mr Trump’s presidency – his defence of white nationalists by suggesting “both sides” were to blame for the violence that claimed the life of an anti-racist demonstrator there in August 2017.
The president on Friday defended his original answer, insisting that he wasn’t sticking up for white supremacists but those who were protesting against the removal of a statue of Confederate Gen Robert E Lee.
But protesters who descended on Charlottesville two summers ago did so chanting white nationalist slogans and carrying Confederate flags, and were joined by former Ku Klux Klan grand wizard David Duke.
There was more bloodshed this weekend as a shooter killed one woman and injured a rabbi and two other people at the Chabad of Poway synagogue near San Diego.
And Pittsburgh was the site of worse earlier shooting, in which 11 people were killed at the Tree of Life Synagogue on October 27.
Mr Trump has condemned both tragedies and has repeatedly griped that he is unfairly blamed for violence by white supremacists.
Mr Biden briefly mentioned the California shooting.
“We’re reminded again because we are in a battle," he said. "We are in a battle for America’s soul and I really believe that."
