The president's personal attorney tells NBCU brass that legal proceedings could be pursued if there's no retraction or apology for stating that "Russian oligarchs" co-signed loans to Trump.

President Donald Trump is ratcheting up his war with the media, as if such thing was conceivable. On Wednesday, his personal attorney Charles Harder threatened NBCUniversal with a defamation suit over what was broadcast the previous night on MSNBC's The Last Word With Lawrence O’Donnell. A missive sent to NBCU headquarters, obtained by The Hollywood Reporter, also addresses a related tweet.

According to Harder's demand letter, "The Program and Tweet make the false and defamatory statements that 'Russian oligarchs' co-signed loans provided to Mr. Trump by Deutsche Bank, and described these 'co-signers' as 'Russian billionaires close to Vladimir Putin.'”

Harder is also warning others against republishing the allegedly false statement. (The Hollywood Reporter is doing so because it believes such republication is necessary to convey what is a matter of utmost public concern and protected by the First Amendment in connection with anticipated litigation.)

In a tweet following Harder's letter, O'Donnell wrote, "Last night I made an error in judgment by reporting an item about the president’s finances that didn’t go through our rigorous verification and standards process. I shouldn’t have reported it and I was wrong to discuss it on the air. I will address the issue on my show tonight."

Later on the show, he retracted his report and apologized.

Trump is certainly notorious for defamation threats, but this may be the first formal one from a sitting president. Should Trump actually pursue a defamation suit and it survived any initial First Amendment challenge, he'd subject himself to a discovery process months after the conclusion of the investigation by Robert Mueller — which Trump repeatedly characterized as a "witch hunt."

Nevertheless, even though Trump has previously claimed not watching MSNBC anymore, O'Donnell's comment appears to have gotten under his skin.

On his Tuesday night show, O'Donnell told colleague Rachel Maddow that a "single source close to Deutsche Bank has told me that the Trump — Donald Trump’s loan documents there show that he has co-signers. That’s how he was able to obtain those loans. And that the co-signers are Russian oligarchs.”

O'Donnell then said, "That would explain, it seems to me, every kind word Donald Trump has ever said about Russia and Vladimir Putin, if true, and I stress the 'if true' part of this.”

In his letter (read in full here), Harder characterizes these allegations as extremely damaging to Trump.

"The only borrowers under these loans are Trump entities, and Mr. Trump is the only guarantor," continues the demand letter. "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."

The discussion of loans from Deutsche Bank come as Democrats in the House of Representatives attempt to subpoena documents from the financial institution. The probe has Democrats and Trump fighting in court. The president's team asserts there's no legitimate legislative purpose for those document requests. The suggestion from the other side is that it could aid impeachment proceedings. On Tuesday, Deutsche Bank confirmed in a court filing that its tranche of Trump-related documents include tax returns.

MSNBC declines comment. Deutsche Bank has also declined comment on O'Donnell's assertion.

Trump is demanding that O'Donnell and NBCU "immediately and prominently retract, correct and apologize for the aforementioned false and defamatory statements."

"Failure to do so will leave my clients with no alternative but to consider their legal options which could include immediate legal proceedings against Mr. O’Donnell and NBCU," concludes the letter. "Should that occur, my clients would pursue all available causes of action and seek all available damages and other legal remedies to the maximum extent permitted by law."

Upon O'Donnell's retraction and apology, Trump accepted it as gracefully as usual.

Trump tweeted: "Crazy Lawrence O’Donnell, who has been calling me wrong from even before I announced my run for the Presidency, even being previously forced by NBC to apologize, which he did while crying, for things he said about me & The Apprentice, was again forced to apologize, this time .for the most ridiculous claim of all, that Russia, Russia, Russia, or Russian oligarchs, co-signed loan documents for me, a guarantee. Totally false, as is virtually everything else he, and much of the rest of the LameStream Media, has said about me for years. ALL APOLOGIZE!"

Jeremy Barr contributed to this report.

Aug. 28, 1 pm PST: O'Donnell's tweet responding to the developing controversy has been added.

Aug. 29, 2 am PST: O'Donnell's follow-up retraction and apology added, plus Trump's reaction.