Republican presidential candidate Donald Trump, shown on the campaign trail in February, spent more than $1 million in legal fees pursuing a libel lawsuit against a book author, which he lost. (Jabin Botsford/The Washington Post)

Donald Trump said last month that he wants to “open up” the nation’s libel laws to make it easier for public figures to sue the news media. The Republican Party’s leading candidate may have been speaking from personal experience: Trump has already sued a journalist for libel — and lost in humiliating fashion.

Trump went to court in early 2006, claiming that he had been libeled in the book “TrumpNation: The Art of Being The Donald,” by Timothy L. O’Brien, then a business reporter at the New York Times. The book briefly addressed Trump’s claims about his net worth, which was then, as it is now, the subject of a great deal of bluster, speculation and opacity.

O’Brien concluded that Trump was worth substantially less than what Trump publicly claimed, an assertion that prompted the business mogul to sue O’Brien and his publisher, Warner Books. He claimed harm to his business and sought $5 billion in damages.

The lawsuit is one of many that Trump has leveled at adversaries and former business partners over the years. But this one may have gone straight to the heart of Trump’s “brand.” Trump pursued it for five years, spending more than $1 million in legal fees, apparently to protect a fundamental aspect of his identity and mythos: That he is not merely very rich but clearly, most sincerely, super rich.

More than a decade later, the issue still clearly stings Trump. O’Brien, he said in an interview Tuesday, “is a whack job, a total nut job . . . one of the sleaziest people I’ve ever done business with. He wrote a book knowing it was totally false. He didn’t know what the assets were, and he disregarded their value. He really did set out with the intent to harm.”

To Trump’s great exasperation, O’Brien showed that there are good reasons to doubt Trump’s assertion that he was worth “five to six billion” dollars in 2005. (In a campaign-disclosure form filed in July, Trump claimed he is now worth more than $10 billion.) Based on documents and interviews with Trump and his associates, O’Brien estimated that Trump had inflated his bankroll as much as 20 times over. Subtracting debts and other liabilities, O’Brien says, Trump’s net worth was pegged at $150 million to $250 million, based on estimates by people with direct knowledge of Trump’s finances.

The suit may have settled the basic question — what is Trump worth? — but the public never got a glimpse of the answer. Trump supplied records and documents, including tax returns that he has declined to release during his campaign, but those were sealed by the court.

In any case, Trump was unable to show that O’Brien had acted with “reckless disregard” for the truth, the standard for sustaining a libel claim against a public figure.

“We blew him up on the whole notion that I set out with reckless disregard and malice,” says O’Brien, now the editor of Bloomberg View. “My lawyers drew and quartered him” on that issue.

The limited public record of the lawsuit includes interesting revelations. One is Trump’s admission, under questioning from O’Brien’s attorneys during a deposition, that he relied on his own “feelings” to assess the value of his holdings.

An attorney asked: Feelings?

“Yes, even my own feelings as to where the world is, where the world is going, and that can change rapidly from day to day,” Trump said, according to the court record. “Then you have a September 11th, and you don’t feel so good about yourself and you don’t feel so good about the world and you don’t feel so good about New York City. Then you have a year later, and the city is as hot as a pistol. Even months after that it was a different feeling. So yeah, even my own feelings affect my value to myself.”

The statement effectively validated O’Brien’s skeptical take on Trump’s self-reporting, which O’Brien characterized as Trump’s “verbal billions.”

A superior court judge in New Jersey ruled that Trump had no case and dismissed his suit in 2009. An appeals court affirmed that decision two years later.

Ultimately, Trump rationalized his defeat this way: “Essentially the judge just said, ‘Trump is too famous,’ ” he told the Atlantic magazine in 2013. “ ‘He’s so famous that you’re allowed to say anything you want about him.’ Well, I disagree with that.”

Well, not exactly.

Both courts cited a lack of “clear and convincing” evidence to satisfy the basic legal test for libeling someone as well known as Trump: willful disregard for the truth. The appeals court noted O’Brien’s diligent and extensive efforts to research Trump’s wealth.

Trump said in an interview that he knew he couldn’t win the suit but brought it anyway to make a point. “I spent a couple of bucks on legal fees, and they spent a whole lot more. I did it to make his life miserable, which I’m happy about.”

O’Brien notes that his reporting on Trump’s wealth consisted of only a few pages of his 288-page book, which was rife with passages about Trump’s “checkered” business career.

So why did Trump ignore the rest and take such offense at the net-worth discussion?

“It’s a measure of his deep insecurity,” O’Brien said. “His wealth and the size of his wealth . . . are integral to how he wants people to perceive him. He looks at the Forbes 400 [list of wealthiest Americans] as the pecking order, and his ego and standing are wrapped up in it. People who are comfortable with their wealth don’t need to brag about it. He’s not in that category.”

As it happens, Trump’s lawsuit and the publicity surrounding it did little to help “TrumpNation.” The book “didn’t sell particularly well,” O’Brien said.

Trump takes some credit for that. “I didn’t read the book,” he said. “I didn’t have time to read it. What I did do was make sure people knew it was false.”

But the author did get one last laugh. In a two-can-play-at-this-game column last year, he facetiously asserted that he, like Trump, was personally worth $10 billion. How did he make such an extraordinary figure? Among other things, O’Brien assessed his home at $6 billion, his aging Ford Escape at $3 billion, and his son’s Pokemon card collection at $100 million.

Absurd, yes. But that’s how rich he personally felt at the time.