US President Donald Trump has lashed out at congressional Democrats after they vowed to summons the White House to produce documents this week.
Committees are demanding documents relating to the administration's dealings with Ukraine, which is now at the heart of an impeachment inquiry.
The president accused Democratic leaders of dishonesty and even treason.
Democrats have defended the inquiry - which focuses on a phone call between Mr Trump and the Ukrainian president.
Warning: this report contains strong language
What did President Trump say?
During a joint news conference with Finnish President Sauli Niinistö, Mr Trump called Mr Biden and his son, Hunter, "stone-cold corrupt".
Mr Trump directed much of his anger towards House Intelligence Committee Chairman Adam Schiff, calling him "shifty Schiff, "a lowlife" and saying he "should resign from office in disgrace".
He added: "Frankly, they should look at him for treason."
Mr Trump also stated that he believed Mr Schiff had "helped write" a complaint about the call filed by a whistleblower, without offering evidence.
The US president told reporters that only "legitimate" whistleblowers should be protected.
"This country has to find out who this person was, because that person's a spy, in my opinion," Mr Trump said.
He labelled the entire inquiry a "hoax" and a "fraudulent crime on the American people" while maintaining he would "always co-operate" with Congress.
The US president also sparred at the White House with a Reuters correspondent, who asked him what he considered treasonous.
As the Finnish leader looked on, Mr Trump said "there are those who think I'm a very stable genius" and said he "probably will be bringing a lot of litigation" against those who participated in the Russia investigation.
When the reporter pressed Mr Trump, the US president cut him off, saying: "Don't be rude."
Earlier, Mr Trump raged at the most powerful elected Democrat, House of Representatives Speaker Nancy Pelosi, and Mr Schiff on Twitter, accusing Democrats of focusing on "BULLSHIT".
The Do Nothing Democrats should be focused on building up our Country, not wasting everyone’s time and energy on BULLSHIT, which is what they have been doing ever since I got overwhelmingly elected in 2016, 223-306. Get a better candidate this time, you’ll need it!— Donald J. Trump (@realDonaldTrump) October 2, 2019
Mr Trump said Mrs Pelosi should focus on her own city, San Francisco, which he described as a "tent city" of homeless people.
What's the inquiry about?
The impeachment inquiry - which may seek to remove Mr Trump from office - stems from the whistleblower's complaint about his 25 July phone conversation with President Volodymyr Zelensky.
In the call, Mr Trump pressed for an investigation of his Democratic political rival Joe Biden and his son, who worked for a Ukrainian gas firm that faced allegations of corruption.
As vice-president and the Obama administration's point man on Ukraine, Mr Biden threatened to withhold US loan guarantees to the country unless it fired its top prosecutor, at a time when Hunter Biden's employer had been under official scrutiny.
But European countries and international bodies had also wanted Viktor Shokin fired, accusing him of failing to clamp down on corruption.
The Ukrainian prosecutor general who took over from Mr Shokin has told the BBC there was no evidence of wrongdoing by Joe or Hunter Biden.
However, questions were raised at the time about a potential conflict of interest for the US vice-president.
Democrats have pointed out Mr Trump's call with the newly elected Mr Zelensky took place at the same time as the US president decided to withhold military aid from Ukraine.
From what has been shared of the call so far, Mr Trump did not directly reference withholding aid, but a full verbatim transcript has not been made available.
Mr Trump's opponents say he was pressuring a vulnerable American ally to interfere in the 2020 US election for the US president's personal benefit.
What did Democrats say?
Democrats have accused the White House of blocking congressional inquiries and refusing to respond to record requests, which has prompted the subpoena threat this week.
House oversight committee chairman Elijah Cummings said in a memo: "I do not take this step lightly.
"Over the past several weeks, the committees tried several times to obtain voluntary compliance with our requests for documents, but the White House has refused to engage with - or even respond to - the committees."
The subpoena will request documents on Mr Trump's call with Ukraine and any related items from acting White House chief of staff Mick Mulvaney.
Mrs Pelosi and Mr Schiff held a joint news conference on Wednesday, defending the impeachment proceedings.
"We're not fooling around here," Mr Schiff said, adding that Democrats did not want the inquiry to "drag on".
He also criticised the president's comments against the whistleblower as "a blatant effort to intimidate witnesses" and "an incitement of violence".
Mr Schiff has also issued a statement saying his committee never reviewed or received the whistleblower's complaint in advance, as Mr Trump claimed.
Quick facts on impeachment
Impeachment is the first part - the charges - of a two-stage political process by which Congress can remove a president from office
If the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial
A Senate vote requires a two-thirds majority to convict - unlikely in this case, given that Mr Trump's party controls the chamber
Only two US presidents in history - Bill Clinton and Andrew Johnson - have been impeached but neither was convicted and removed
President Nixon resigned before he could have been impeached