With control over some of President Trump’s most prized real estate holdings in jeopardy, Mr. Trump says he will make a rare, voluntary trip to court at New York on Monday for the start of a civil trial in a lawsuit that already has resulted in a judge ruling that he committed fraud in his business dealings.

“I’m going to Court tomorrow morning to fight for my name and reputation,” Mr. Trump wrote Sunday night on his Truth Social platform.

Mr. Trump’s move comes as one real estate broker in Miami used the word “ridiculous” to describe to the Sun an $18 million valuation on one Trump property, Mar-a-Lago at Palm Beach, cited by the judge sitting on the case in New York. The low valuation has surfaced questions about whether the jurist correctly comprehended Mar-a-Lago’s market value.  

“THIS WHOLE CASE IS SHAM,” Mr. Trump exclaimed. “See you in Court — Monday morning.”

The trial is the culmination of a years-long investigation by New York’s attorney general, Letitia James, who accused Mr. Trump and his company of habitually lying about his wealth in financial statements.

Last week, Judge Arthur Engoron resolved the lawsuit’s top claim before the trial even began, ruling that Mr. Trump routinely deceived banks, insurers, and others by exaggerating the value of assets on paperwork used in making deals and securing loans.

The 45th president and a who’s who of people in his orbit — his two eldest sons, Trump Organization executives, and a former lawyer turned foe, Michael Cohen — are all listed among dozens of potential witnesses.

Mr. Trump isn’t expected to testify for several weeks. His trip to court Monday will mark a remarkable departure from his past practice.

Mr. Trump didn’t come to court as either a witness or a spectator when his company and one of its top executives was convicted of tax fraud last year. He didn’t show, either, for a trial earlier this year in which a jury found him liable for sexually assaulting a writer, E. Jean Carroll, in a department store dressing room.

In some ways, though, this new trial comes with higher stakes. General James, a Democrat, is seeking $250 million in penalties and a ban on doing business in New York.

Judge Engoron’s ruling of last week, if upheld on appeal, would also shift control of some of Mr. Trump’s companies to a court-appointed receiver and could force him to give up prized New York properties such as Trump Tower, a Wall Street office building, golf courses, and a suburban estate.

Mr. Trump called it a “a corporate death penalty.”

“I have a Deranged, Trump Hating Judge, who RAILROADED this FAKE CASE through a NYS Court at a speed never before seen,” Mr. Trump wrote on his Truth Social platform. In his post Sunday night, Mr. Trump wrote that Judge Engoron is “unfair, unhinged, and vicious in his PURSUIT of me.”

Judge Engoron will decide on six remaining claims in General James’s lawsuit, including allegations of conspiracy, falsifying business records, and insurance fraud.

General James’s lawsuit accused Mr. Trump and his company of a long list of fibs in the financial statements he gave to banks. In a recent court filing, Ms. James’s office alleged that Mr. Trump exaggerated his wealth by as much as $3.6 billion.

Among the allegations were that Mr. Trump claimed his Trump Tower apartment in Manhattan — a three-story penthouse replete with gold-plated fixtures — was nearly three times its actual size and worth an astounding $327 million. No apartment in New York City has ever sold for close to that amount, Ms. James said.

Mr. Trump valued Mar-a-Lago as high as $739 million — more than 10 times a more reasonable estimate of its worth, Ms. James claimed. Mr. Trump’s figure for the private club and residence was based on the idea that the property, now a private club, could be developed for residential use, but deed terms prohibit that, Ms. James said.

Mr. Trump has denied wrongdoing, arguing in sworn testimony for the case that it didn’t matter what he put on his financial statements because they have a disclaimer that says they shouldn’t be trusted.

Mr. Trump and his lawyers have also argued that no one was harmed by anything in the financial statements. Banks he borrowed money from were fully repaid. Business partners made money. And Mr. Trump’s own company flourished.

Ms. James’s lawsuit is one of several legal headaches for Mr. Trump as he campaigns for a return to the White House in next year’s election. He has been indicted four times since March, accused of plotting to overturn his 2020 election loss, hoarding classified documents, and falsifying business records related to hush money paid on his behalf.

The trial could last into December, Judge Engoron said.