MSNBC nighttime host Lawrence O'Donnell retracted a bombshell report on Wednesday, admitting he shouldn't have aired a claim that Russian oligarchs co-signed Donald Trump's loans with Deutsche Bank

O'Donnell walked his reporting back in a tweet that came after the president's personal lawyer threatened the cable network with a defamation lawsuit.

He teased his news shocker Tuesday night at the end of Rachel Maddow's hour of programming, inspiring a torrent of anti-Trump tweets that included the hashtag #RussianCosigners – which quickly rose to #2 on Twitter's 'trending' chart in the United States.

O'Donnell had said a single source had told him information that 'goes a few very important steps beyond' information that was publicly available about Trump's lending history with the German bank.

'The source says that Deutsche Bank is in possession of loan documents that show Donald Trump has obtained loans with co-signers, and that he would not have been able to obtain those loans without co-signers,' said O'Donnell. 'The source close to Deutsche Bank says that the co-signers of Donald Trump's Deutsche Bank loans are Russian billionaires close to Vladimir Putin.'

MSNBC nighttime host Lawrence O'Donnell retracted a story on Wednesday that had drawn a defamation lawsuit threat from President Donald Trump's personal lawyer

O'Donnell (right) stunned fellow host Rachel Maddow on Tuesday by reporting that Russian oligarchs had co-signed President Trump's loans with Deutsche Bank

O'Donnell also teased the news with a tweet Tuesday night before he went on the air; Trump's personal lawyer threateded a lawsuit on Wednesday

The president hasn't addressed the rumor personally, leaving it hanging like an unlikely rumor that his opponents have seized on nonetheless

O'Donnell cast that apparent blockbuster in a political light after Maddow gasped 'What? Really?'

'If true,' he said, 'that would explain every kind word Donald Trump has ever said about Russia and Vladimir Putin.'

'If true, that would be a significant factor in Vladimir Putin's publicly stated preference for presidential candidate Donald Trump over presidential candidate Hillary Clinton,' O'Donnell added.

The host of 'The Last Word' teased his revelation with a tweet that quickly went viral. But he was hedging by the end of his hour-long program.

'Now, I want to stress, that's a single source,' he said. 'This has not been confirmed by NBC News. I have not seen any documentation from Deutsche Bank that supports this and verifies this. This is just a single source who has revealed that to me. And that's where that stands at this point. It's going to need a lot more verification before that can be a confirmable fact.'

Trump's eldest son Don Jr. tweeted an accusation that O'Donnell 'totally fabricated' the story

MSNBC's 'Morning Joe' producers were under pressure by early Wednesday morning to confirm or disprove O'Donnell's story. Nearly two hours into the broadcast, booking producer Michael Del Moro conceded on Twitter that the lone source hadn't seen any loan paperwork.

'Deutsche Bank is declining to comment on Lawrence O’Donnell’s reporting that Russian oligarch’s [sic] co-signed Trump’s loans,' Del moro wrote.

'The information came from a single source who has not seen the bank records. NBC has not seen those records and has not yet been able to verify the reporting.'

Donald Trump Jr. tweeted sarcastically that the network's inability to confirm O'Donnell's report is '[l]ileky because he totally fabricated it.'

Charles Harder, the president's personal lawyer, threatened a lawsuit in a two-page letter that demanded an immediate retraction

By Wednesday morning one MSNBC producer was tweeting that O'Donnell's source had not personally seen any of Trump's bank records, reducing the story to the status of a rumor

Trump's attorney, Charles Harder, wrote to MSNBC executives on Wednesday, complaining about 'false and defamatory statements.'

Harder wrote that there are no co-signers on Trump's Deutsche Bank loans.

'The only borrowers under these loans are Trump entities, and Mr. Trump is the only guarantor,' he declared. 'Numerous documents for each of these loans are also recorded, publicly available and searchable online.'

'Thus, actual malice can easily be proven based on your reckless disregard of the truth and unreasonable reliance on an alleged "source" who you will not even identify in your story and likely is seeking to mislead you and the public for political reasons or other ulterior motives.'

'Actual malice' is a legal standard that applies to public figures who want to argue that they have been libeled or slandered. It requires proving that a defendant was reckless and published a lie despite knowing, or being expected to know, that it was false.