Joe Biden has made his first call for Donald Trump’s impeachment and told a rally the president has “already convicted himself”.
The former US vice-president is one of the Democratic front-runners in the 2020 presidential race.
His party has launched an impeachment inquiry over claims that Mr Trump illegally pressured a foreign leader to investigate Mr Biden and his son.
Mr Trump denies wrongdoing and has dismissed the probe as a “witch hunt”.
On Tuesday, the White House said it would not co-operate, arguing that the proceedings were unconstitutional.
Addressing supporters in New Hampshire, Mr Biden said the president had already “violated his oath of office” and “indicted himself by obstructing justice, refusing to comply with a congressional inquiry”.
It was his most forceful response to date.
Previously, the former vice-president had only gone so far as to back an inquiry to gather the facts, even as his top Democratic rivals were calling for impeachment.
Shortly after Mr Biden spoke, Mr Trump hit back at “Sleepy Joe Biden” and his “failing campaign”.
The Democratic-led inquiry is trying to establish whether the Republican president withheld nearly $400m (£327m) in aid to nudge Ukraine’s president into launching an inquiry into Mr Biden and his son.
Hunter Biden was on the board of a Ukrainian energy company, Burisma.
In a phone call on July 25, Mr Trump asked the newly elected Volodymyr Zelensky to scrutinise the former US vice-president.
A whistleblower raised concerns about the phone call, and the Democratic leader of the House of Representatives, Speaker Nancy Pelosi, announced a formal impeachment investigation last month.
When Hunter Biden joined the firm in 2014, questions were raised about a potential conflict of interest for his father.
But there is no evidence of wrongdoing on the part of the Bidens.
On Tuesday, the White House released an eight-page letter sent to top Democrat and House Speaker Nancy Pelosi, calling the inquiry “constitutionally invalid”.
It continued: “In order to fulfil his duties to the American people… President Trump and his Administration cannot participate in your partisan and unconstitutional inquiry under these circumstances.”
Ms Pelosi dismissed the letter as “manifestly wrong”. -BBC