In the memo, which Comey shared with top FBI associates, he wrote that Trump said, "I hope you can let this go," referring to the inquiry into Flynn's actions. "I hope you can see your way clear to letting this go, to letting Flynn go," said Trump, according to a source who read the memo. "He is a good guy. I hope you can let this go."
Multiple sources who worked closely with Comey, including at the Justice Department, say he is known for his contemporaneous and thorough note-taking. "He documents everything," one source said.
ABC News has not seen the memo.
What sources say is in the memo
Trump's alleged request to Comey about the Flynn investigation came the day after Flynn was forced to resign after misleading the administration about his contacts with Russian officials. The FBI, which is investigating Russian interference in last year's presidential election, declined to comment on the story.
The newest allegations, which the White House strenuously denied, came as the Trump administration was still reeling from a Washington Post report on Monday that Trump shared classified information with Russian officials during a meeting at the White House last week.
Responding to the Comey memo reports, the White House on Tuesday said that "while the president has repeatedly expressed his view that Gen. 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 Gen. Flynn."
"The president has the utmost respect for our law enforcement agencies and all investigations," the White House statement added. "This is not a truthful or accurate portrayal of the conversation between the president and Mr. Comey."
A White House official emphasized that in testimony before the Senate Intelligence Committee last week, acting FBI Director Andrew McCabe said, "There has been no effort to impede our investigation to date."
What the reaction has been
The Comey memo revelation was met with alarm on Capitol Hill on Tuesday night, not only among Democrats but among several Republicans as well.
"If these reports are true, the president's brazen attempt to shut down the FBI's investigation of Michael Flynn is an assault on the rule of law that is fundamental to our democracy," said House Minority Leader Nancy Pelosi, D-Calif. "At best, President Trump has committed a grave abuse of executive power. At worst, he has obstructed justice."
"The country is being tested in unprecedented ways. I say to all my colleagues in the Senate, history is watching," tweeted Senate Minority Leader Chuck Schumer of New York.
"The memo is powerful evidence of obstruction of justice and certainly merits immediate and prompt investigation by an independent special prosecutor," said Sen. Richard Blumenthal, D-Conn.
The reactions on the Republican side, while more muted, nonetheless expressed serious concern.
"Congress needs to see the Comey memo," tweeted Sen. Jeff Flake, R-Ariz.
Sen. Lindsey Graham, R-S.C., said, "If Mr. Comey was alleging the president did something inappropriate, it's an open invitation to come to the Judiciary Committee and tell us about it. I don't want to read a memo. I want to hear it from him."
A spokesperson for Republican Sen. Richard Burr, the chairman of the Senate Intelligence Committee, which is leading an investigation into possible Trump associates' connections to Russia during the 2016 election season, said in a statement, "The committee relies on facts to guide the investigation ... Sen. Burr will follow relevant leads, but the committee has not seen what The New York Times reported today. It certainly raises questions, and he will follow up on acquiring those facts from credible sources."
On Tuesday evening, Rep. Jason Chaffetz, R-Utah, the chairman of the House Oversight Committee, sent a letter to McCabe requesting that the FBI provide "all memoranda, notes, summaries and recordings referring or relating to any communications between Comey and the president" by May 24. Earlier in the day, Chaffetz tweeted that he has his "subpoena pen ready."
Responding to the Chaffetz letter, House Speaker Paul Ryan's spokeswoman AshLee Strong said, "We need to have all the facts, and it is appropriate for the House Oversight Committee to request this memo."
All 33 Democrats on the House Oversight and Judiciary committees penned letters to the chairmen of both committees demanding an investigation into the actions of Trump, Attorney General Jeff Sessions and top White House aides and whether they "engaged in an ongoing conspiracy to obstruct the criminal, counterintelligence and oversight investigations" into the Trump campaign at the Justice Department and on Capitol Hill.
They also requested an immediate public hearing with Comey and copies of all his memos and records relating to Trump.
Wednesday morning, the Senate Intelligence Committee sent a letter formally asking Comey to appear in open and closed sessions before the committee. The Senate Judiciary Committee made a similar request later in the day.
The Judiciary Committee letter to McCabe said requested “all such memos, if they exist, that Mr. Comey created memorializing interactions he had with Presidents Trump and Obama, Attorneys General Sessions and Lynch, and Deputy Attorneys General Rosenstein, Boente, and Yates regarding the investigations of Trump associates’ alleged connections with Russia or the Clinton email investigation.” A deadline of May 24 was set for the information.
The committee also sent a second letter to McCabe seeking any notes or memos prepared by Comey related to any communications he may have had with White House or DOJ officials regarding the Russia investigation.
Trump fired Comey — who was not yet four years into a 10-year term — last week, and the president admitted later he was thinking about the Russia investigation when he made the decision. The White House originally pegged his decision on recommendations from Sessions and Deputy Attorney General Rod Rosenstein, but Trump pushed back, saying it was his decision alone.