WILMINGTON, DELAWARE (WASHINGTON POST) - Former United States vice-president Joe Biden called on Congress to begin impeachment proceedings against President Donald Trump if the White House continues to stonewall Congress' investigations, including into reports that Mr Trump asked Ukraine's President to investigate Mr Biden and his son, Hunter.

"I can take the political attacks. They'll come and they'll go, and in time they'll soon be forgotten," Mr Biden said in brief remarks here on Tuesday afternoon that underscored his cautious approach to the spectre of impeachment.

"But if we allow a president to get away with shredding the United States Constitution, that will last forever."

Mr Biden, who spoke for several minutes and left without taking questions, said if Mr Trump doesn't comply with Congress, it will be forced to begin impeachment hearings.

"That would be a tragedy," he said. "But a tragedy of his own making."

Mr Biden's remarks - his most extensive on the topic since news emerged last week that a whistle blower raised alarm about Mr Trump's request for an investigation into Mr Biden - came on an extraordinary day of developments.

Soon after Mr Biden spoke, House Speaker Nancy Pelosi, a California Democrat, announced the opening of a formal impeachment inquiry, effectively moving beyond Mr Biden's demands.

Mr Biden's own remarks fell short of asking for impeachment proceedings to begin now - a position that many of his Democratic rivals arrived at months ago - and were largely consistent with his past statements.

Congress already is undertaking many investigations into Mr Trump, his administration, his campaign and his private businesses. The White House has defied subpoenas for testimony and documents.

The latest moves stemmed from a decision by the whistle blower within the intelligence community to file a complaint alleging that Mr Trump had asked for an investigation of the Bidens during a July phone conversation with Ukrainian President Volodymyr Zelensky.

Mr Trump, who had put a hold on military aid for Ukraine about a week before the call, allegedly brought up Mr Biden's role in pressing for the firing of a top prosecutor in Ukraine in 2016.

Mr Biden's son, Hunter, sat on the board of a Ukrainian gas company that had been investigated by the prosecutor. Mr Biden's effort to get the prosecutor removed, however, was related to the Obama administration's belief that the prosecutor was ineffective at rooting out corruption in Ukraine.

The tentative nature of Mr Biden's remarks underscored the difficulty in predicting how the coming national maelstrom will affect his candidacy.

The moment brings significant opportunities for Mr Biden to showcase that he is Mr Trump's primary foil, and his allies hope it will cause the party to rally behind him even amid a fractious Democratic primary process.

But it also creates the potential for political turmoil, with a daily onslaught of accusations - many of them unfounded and dismissed by fact-checkers - as Mr Trump and his allies seek to focus as much on Mr Biden as Democrats do on the President.

Many Democrats already are worried about reliving the way Mrs Hillary Clinton was attacked in 2016, and fear Mr Biden's campaign could stumble in much the same way hers did while facing Mr Trump's aggressive tactics.

Mr Trump's request for Ukraine to investigate the Bidens would be at the centre of any impeachment proceedings.

And while there has been no evidence of wrongdoing, a months-long investigation by Congress would bring about frequent reminders that Mr Biden's son was earning money from a Ukrainian gas company while his father, as vice-president, was overseeing diplomatic efforts in the country.

The impeachment proceedings are also ones in which several of Mr Biden's primary rivals - the list currently includes six senators and two members of Congress - could take part.

Mr Biden previously called on Mr Trump to release a transcript of his conversation with the Ukrainian President, which Mr Trump said he would do on Wednesday.

On Tuesday, Mr Biden also called on Mr Trump to release a copy of the formal complaint made by the whistle blower, which congressional overseers have been blocked from receiving.

Mr Biden is mostly alone in the 2020 Democratic presidential field in not calling for the House to begin an immediate impeachment inquiry.

"We appear to have a president who, unbelievably, may have used security funds - funds designed to protect the security of the people of the United States - as a means to gain political dirt on an opponent," Senator Bernie Sanders, a Vermont independent, said on Tuesday at a campaign stop in Davenport, Iowa, calling for the House to begin impeachment proceedings now.

But Mr Sanders would not say whether he would vote to convict Mr Trump if articles of impeachment were before the Senate, explaining that he wanted to review evidence before concluding how he would vote.

"You're asking me to tell you how I'm going to vote before I hear the evidence coming before the Senate," he said. "And that would be irresponsible."

Mr Sanders reserved judgment about whether Mr Hunter Biden's conduct is fair game.

"I don't know enough at this point to make any definitive statement," he said.

Senator Elizabeth Warren, a Massachusetts Democrat, was the first of the 2020 candidates to call for Mr Trump's impeachment, after the release of then-special counsel Robert Mueller's Russia investigation report.

"I called for impeachment five months ago, the day after the Mueller report came out. Trump continues to commit crimes because he believes he's above the law. If Congress does nothing to respond, he'll be right. We must begin impeachment proceedings - now," Ms Warren tweeted on Tuesday.

South Bend, Indiana, Mayor Pete Buttigieg, who has been more cautious around the question of impeachment, told MSNBC on Monday that Mr Trump has "made it clear he deserves to be impeached" after the most recent reports of his conversation with Mr Zelensky.

Entrepreneur and presidential hopeful Andrew Yang wrote on Twitter: "Given the President's latest actions, I think impeachment is the right path forward."

Former congressman Beto O'Rourke wrote: "To my former colleagues in Congress: Finish the job and impeach him."

Mrs Clinton also weighed in, telling People magazine that she was now in favour of moving towards impeachment of a president who she called a "corrupt human tornado".

"I did not come to that decision easily or quickly, but this is an emergency as I see it," she said.

Referencing Mr Trump's targeting of Mr Biden, she added: "If that's not an impeachable offence, I don't know what is."

Mr Biden spoke only obliquely on Tuesday about the fact that he was Mr Trump's target, and he never mentioned his son specifically.

"I knew, when I decided to run, this President would attack me and anyone else who he thought would be a threat to his winning again," Mr Biden said. "And know that, even though every reputable publication that has looked at the charge that has been made against me, and they found them baseless and untrue and without merit, that's not about to stop him."

Although Mr Biden's rhetoric escalated, his position was largely the same as it has been for months.

During a June 4 event in New Hampshire, Mr Biden was asked by an 11-year-old girl if he supported impeaching Mr Trump.

"If the President does not soon start to cooperate with the legitimate investigations of the Congress, then in fact the Congress has no option but to begin an impeachment inquiry," Mr Biden said.

"I am not looking forward to an impeachment process - I really mean it. I think it'll be a gigantic distraction on things that, in fact, we should be focusing on getting done."