Lawyers for President Donald Trump wasted no time firing off a demand letter after MSNBC's Lawrence O'Donnell on Tuesday night made allegedly "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.'"

"If true, that explains every kind word Trump has ever said about Russia and Putin," O'Donnell wrote in a follow-up tweet.

Media law ace Charles Harder of Harder LLP demanded a retraction, correction and apology, asserting that the statements were made with actual malice and reckless disregard for the truth.

O'Donnell quickly backed down, though he didn't immediately retract the story, which relied on a single, unnamed source.

On Wednesday, the MSNBC host tweeted, "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."

On air last night, he formally retracted the story. "We don't know whether the information is inaccurate, but the fact is we do know it wasn't ready for broadcast, and for that I apologize."

Will that be enough to mollify the president? Harder did not respond to a request for comment, but O'Donnell and NBC have some cause for concern.

Harder has an impressive track record when it comes to winning big-time defamation cases.

In 2016, along with co-counsel Ken Turkel and Shane Vogt, he won a $140 million jury verdict for Hulk Hogan after Gawker Media posted a sex tape of the professional wrestler.

In 2017, Harder won a reported $2.9 million settlement on behalf of Melania Trump, who sued U.K's Daily Mail for libel over an article that alleged she "provided services beyond simply modeling" in the 1990s.

The paper retracted and withdrew the story and apologized to the First Lady.

Last year, Harder scored again, successfully defending the president in a defamation suit brought by Stormy Daniels. Represented by Michael Avenatti, Daniels (who real name is Stephanie Clifford) was ordered to pay Trump $293,000 for attorneys' fees, costs and sanctions.

While Harder has proven he's got the chops to win libel cases, the O'Donnell matter may have some complications.

For one thing, the TV host didn't try to pass off the allegations as hard facts. "I want to stress that is a single source, that has not been confirmed by NBC News," O'Donnell said. "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."

But Harder attacked him for failing to investigate before blurting out the allegations. "Numerous documents for each of these loans are also recorded, publicly available, and searchable online," he wrote. "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."

Of course, the best defense against libel is truth. A look at, say, the president's tax returns might prove who's right and who's wrong. But it's hard to imagine Trump would be keen to open himself up to discovery if this blows up into a full lawsuit.