President Trump’s $500 million lawsuit against one of his former lawyers, Michael Cohen, threatens the Manhattan district attorney’s star witness with economic ruin ahead of Mr. Trump’s criminal trial. It also suggests that with the former president’s liberty on the line, he has determined that his best defense is litigation offensive.  

The suit, a civil action filed in federal court at Miami, alleges “breaches of fiduciary duty, unjust enrichment, conversion, and breaches of contract” on the part of the disbarred lawyer. The complaint accuses Cohen of defaulting on his attorney-client relationship with Mr. Trump, and “spreading falsehoods” that are “likely to be embarrassing or detrimental.”

Mr. Trump argues that Cohen acted with “malicious intent and to wholly self-serving ends” in “disparaging” the former president “through myriad public statements, including the publication of two books, a podcast series, and innumerable mainstream media appearances.” These “illicit acts,” Mr. Trump’s lawyers write, have increased in “frequency and hostility” in recent months.

Cohen pleaded guilty in 2018 to tax fraud and campaign finance violations arising from Mr. Trump’s alleged hush money payments to the porn star Stormy Daniels in advance of the 2016 presidential election. Those payments, which prosecutors — and Cohen himself — explain were processed through Cohen, are the core of District Attorney Alvin Bragg’s case against Mr. Trump. 

Cohen also pleaded guilty to making false statements to a federally insured bank, the most serious crime to which he offered a plea and one that exposed him to a potential 30-year sentence. He went up the river for a shorter stretch, of course. He finished his sentence in 2021, having been sprung on early release.

Since then Cohen has emerged as a vocal critic of Mr. Trump, as well as a witness before the grand jury that decided to hand up charges at New York. Now, Mr. Trump accuses him of aiming to “repair a reputation shattered by his repeated misrepresentations and deceptive acts” by damaging the former president.  

Cohen has evinced a willingness to tell his story at Mr. Trump’s expense, going on the American Broadcasting Company last month to assert that the “facts are the facts. The truth is the truth, and the truth will always rise. So I’m not worried about anything that they want to come at me with.”

Cohen once took a different approach, telling ABC in 2011 that “if somebody does something Mr. Trump doesn’t like, I do everything in my power to resolve it to Mr. Trump’s benefit. If you do something wrong, I’m going to come at you, grab you by the neck, and I’m not going to let you go until I’m finished.”

Mr. Trump’s suit accuses Cohen of damaging his chances of winning the 2020 presidential election, alleging that the timing of the release of his book, “Disloyal,” indicates that Cohen “intended to improperly disclose” Mr. Trump’s “confidences when it would be most lucrative to do so” and with the “most damaging reputational effect.”

Mr. Trump has long been an aggressive litigant. In January, a judge in Florida levied more than $1 million in sanctions against Mr. Trump and one of his attorneys, Alina Habba, stemming from a lawsuit against Secretary Clinton and other officials that the jurist called “completely frivolous, both factually and legally, and which was brought in bad faith for an improper purpose.”