US President Donald Trump has complained that "no politician in history" has been treated worse as pressure mounted over his firing of FBI director James Comey.
Democrats demanded an independent commission to dig into his sacking of Mr Comey, but Republican House Speaker Paul Ryan cautioned against "rushing to judgment".
Mr Ryan said Congress needs to get the facts, but "it is obvious there are some people out there who want to harm the president".
Elijah Cummings, top Democrat on a key House oversight panel, countered that Mr Ryan and the Republicans had shown "zero, zero, zero appetite for any investigation of President Trump".
The White House has denied reports that Mr Trump pressed Mr Comey to drop an investigation into Mr Trump's first national security adviser, Michael Flynn.
In addition Mr Trump is facing pointed questions about his discussions with Russian diplomats during which he is reported to have disclosed classified information.
In an extraordinary turn of events, Russian President Vladimir Putin offered to turn over to Congress records of Mr Trump's discussions with the diplomats.
The White House has played down the importance and secrecy of the information Mr Trump gave to the Russians, which had been supplied by Israel under an intelligence-sharing agreement.
Mr Trump himself said he had "an absolute right" as president to share "facts pertaining to terrorism" and airline safety with Russia.
Yet US allies and some members of Congress have expressed alarm.
Republicans and Democrats alike were eager to hear from Mr Comey, who has increasingly emerged as a central figure in the unfolding drama.
The Senate intelligence committee on Wednesday asked Mr Comey to appear before the panel in both open and closed sessions.
The committee also asked acting FBI director Andrew McCabe to give the committee any notes that Mr Comey might have made regarding discussions he had with White House or Justice Department officials about Russia's efforts to influence the 2016 presidential election.
Mr Putin told a news conference that he would be willing to turn over notes of Mr Trump's meeting with the Russian diplomats if the White House agreed.
He dismissed outrage over Mr Trump's disclosures as US politicians whipping up "anti-Russian sentiment".
Asked what he thinks of the Trump presidency, Mr Putin said it is up to the American people to judge and his performance can be rated "only when he's allowed to work at full capacity", implying that someone is hampering Mr Trump's efforts.
Mr Trump himself has not directly addressed the latest allegations that he pressured Mr Comey to drop the Flynn investigation.
But the swirling questions about his conduct were clearly on his mind when he told graduates at the US Coast Guard Academy in Connecticut that "no politician in history, and I say this with great surety, has been treated worse or more unfairly".
Striking a defiant stance, he added: "You can't let the critics and the naysayers get in the way of your dreams. ... I guess that's why we won. Adversity makes you stronger. Don't give in, don't back down. ... And the more righteous your fight, the more opposition that you will face."
As for Mr Comey, whom Mr Trump fired last week, the FBI director wrote in a memo after a February meeting at the White House that the new president had asked him to shut down the FBI's investigation of Mr Flynn and his Russian contacts, said a person who had read the memo.
The Flynn investigation was part of a broader probe into Russian interference in last year's presidential election.
Mr Comey's memo, an apparent effort to create a paper trail of his contacts with the White House, would be the clearest evidence to date that the president has tried to influence the investigation.
Representative Jason Chaffetz, Republican chairman of the House oversight committee, sent a letter to the FBI on Tuesday requesting that it turn over all documents and recordings that detail communications between Mr Comey and Mr Trump.
He said he would give the FBI a week and then "if we need a subpoena, we'll do it".
John McCain, chairman of the Senate Armed Services Committee, said late on Tuesday that the developments had reached "Watergate size and scale".
Mitch McConnell, the Republican leader of the Senate, said: "It would be helpful to have less drama emanating from the White House."
The existence of the memo was first reported on Tuesday by The New York Times.
The White House vigorously denied it all.
"While the president has repeatedly expressed his view that General Flynn is a decent man who served and protected our country, the president has never asked Mr Comey or anyone else to end any investigation, including any investigation involving General Flynn," a White House statement said.
Mr Trump fired Mr Flynn on February 13, on grounds that he had misled Vice President Mike Pence and other officials about his contacts with Russians.
The intensifying drama comes as Mr Trump is set to embark on Friday on his first foreign trip, which had been optimistically viewed by some aides as an opportunity to reset an administration floundering under an inexperienced president.
Senator Lindsey Graham said: "He's probably glad to leave town, and a lot of us are glad he's leaving for a few days."