Mark Cuban has tussled with Bill O’Reilly, Donald Trump and an array of N.B.A. referees.

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Next up for the billionaire? The federal government.

The Securities and Exchange Commission’s insider trading trial against Mr. Cuban opens in a federal courtroom in Dallas on Monday, the culmination of a five-year battle. Mr. Cuban, a 55-year-old reality TV regular best known for his courtside antics at games of the Dallas Mavericks, the basketball team he owns, is one of the few celebrities to land on the agency’s radar.

Mr. Cuban’s star power, coupled with the S.E.C.’s renewed ambition for taking cases to court, underpins the importance of this trial.

After enduring its share of losses at trial and a public lashing for missing signs of the financial crisis, the agency is fresh off its most significant courtroom victory in recent memory with a win over Fabrice Tourre, a former Goldman Sachs trader at the center of a toxic mortgage deal. A victory in Mr. Cuban’s case might further embolden the S.E.C. as it seeks to hold individuals accountable at trial, a policy championed by its new chairwoman, Mary Jo White.

For Mr. Cuban, who has a net worth pegged at $2.5 billion, the courtroom fight is not about the money. If found liable, he faces a fine of about $2 million. Having doled out about that much in fines to the National Basketball Association, Mr. Cuban is instead fighting to clear his name and dress down an agency that accused him of trading on confidential information when dumping his stake in an Internet company.

Over five years and in three different courthouses, Mr. Cuban’s case has had many twists and turns. A judge dismissed the lawsuit in 2009 and an appeals court reinstated it a year later. This year, Mr. Cuban sought to delay the trial to accommodate what the S.E.C. mocked as his “Hollywood production schedule” — a swipe at his commitment to film the latest season of the reality show “Shark Tank.”

Mr. Cuban has fired his own shots in the case, using his blog to criticize the agency for a “facts be damned” approach, suing it to uncover records about its investigation and even suggesting that the insider trading case stemmed from an S.E.C. employee who questioned Mr. Cuban’s involvement in a documentary film that smeared “the good name of a patriot like President Bush.”

While the employee was fired, an inquiry by the S.E.C.’s inspector general cleared the agency of any misconduct, concluding that the employee was not involved in the investigation of Mr. Cuban.

The contentious back story will not figure into the trial, but Mr. Cuban’s feistiness very likely will. And that personality, which will enter the spotlight when Mr. Cuban takes the witness stand as expected, might sway a jury of Dallas residents who have largely embraced Mr. Cuban.

“He does things I never thought he’d do,” said L.T. Johnson, a Dallas resident and Mavericks fan. “He should be up in a box like the other owners, but instead he’s on the floor, with the players and fans.”

Mr. Cuban made his fortune on the eve of the dot-com crash in 1999, selling his start-up, Broadcast.com, to Yahoo for nearly $6 billion. He bought the Mavericks months later, and now has co-ownership stakes in the movie chain Landmark Theaters, among other media companies.

The S.E.C.’s case traces to June 2004, when the Internet search engine company Mamma.com was planning a private offering of its stock. The company expected Mr. Cuban to balk at the deal, which was likely to hurt Mamma.com’s stock price and dilute the holdings of existing shareholders like Mr. Cuban.

Yet Mamma.com hoped to win over Mr. Cuban, who already owned 6.3 percent of the company’s shares. During an eight-minute phone call on June 28, 2004, Mamma.com’s chief executive, Guy Fauré, made his pitch for the private offering.

According to the S.E.C., Mr. Cuban agreed to keep the information confidential. And after becoming “very upset and angry” when learning about the private offering, the S.E.C. said, Mr. Cuban declared “I can’t sell” the existing shares because he had access to inside information.

Mr. Fauré urged Mr. Cuban to consult the investment bank arranging the private offering, Merriman Curhan Ford, now Merriman Capital, before deciding. When Mr. Cuban spoke to a Merriman employee, according to the S.E.C., he learned “additional confidential details” about the deal.

One minute after the call with Merriman ended, Mr. Cuban ordered his stockbroker to dump his entire stake in Mamma.com. By unloading the shares just hours before Mamma.com announced the offering, Mr. Cuban avoided about $750,000 in losses.

More than four years later, the S.E.C. charged Mr. Cuban with insider trading. Since then, the S.E.C. has argued that Mr. Cuban tried to “conceal his wrongful trading” and concocted “a cover story,” by sending an e-mail to his broker saying, “I want to make sure I was 100 pct kosher on that trade.”

Mr. Cuban’s lawyers dispute those accusations.

“Mr. Cuban’s e-mail to his broker, which asked that the investment bank’s compliance department be consulted, obviously sought more scrutiny of his entirely proper trading, not to cover it up,” the lawyers, Christopher J. Clark of Latham & Watkins, Stephen Best of Brown Rudnick and Thomas M. Melsheimer of Fish & Richardson, said in a statement. They added that “far from concealing his trading, Mr. Cuban voluntarily spoke to the S.E.C. without an attorney when they called him about an unrelated” investigation into Mamma.com.

“We expect that when the facts come out,” the statement said, “Mark will be vindicated.”

The defense has several factors working in its favor. For one, the judge assigned to the case, Sidney A. Fitzwater, has been skeptical of the S.E.C. from the start.

Judge Fitzwater dismissed the case in 2009, ruling that the S.E.C. had to show two things: that Mr. Cuban agreed to keep the information confidential and that he agreed not to trade on it. The S.E.C.’s complaint was “deficient” in proving the second, Judge Fitzwater ruled, noting that Mr. Fauré never asked Mr. Cuban not to trade.

The United States Court of Appeals for the Fifth Circuit reversed the judge’s dismissal a year later. This March, Judge Fitzwater allowed the case to proceed to trial, calling his decision “in some respects a close one.”

The S.E.C. could face additional problems at trial as its case hinges on the testimony, and memory, of Mr. Fauré. There is no recording of Mr. Fauré’s call with Mr. Cuban. And Mr. Cuban does not recall the nine-year-old conversation.

To further bolster the defense, Mr. Cuban’s lawyers are likely to portray Mr. Fauré as unreliable.

During his initial interview with the S.E.C., Mr. Fauré did not mention that Mr. Cuban immediately agreed to keep the information confidential. It was not until his second interview with the S.E.C. that Mr. Fauré recounted how Mr. Cuban said “something to the effect” of “um hum, go ahead” in response to hearing that Mamma had “confidential information” to share.

Mr. Cuban’s lawyers will most likely highlight the curious timing of Mr. Fauré’s additional testimony. Mr. Fauré, according to court records, changed his story about two weeks after learning that the S.E.C. dropped an unrelated investigation into Mamma.com.

The lawyers will also stress that Mr. Fauré’s e-mails to Mr. Cuban did not mention a confidentiality agreement. Mr. Fauré chose not to pursue such an agreement, court records say, even though one of Mamma.com’s outside lawyers advised the company to obtain a signed contract from Mr. Cuban.

If Mr. Cuban escapes unscathed in the courtroom, his problems will very likely shift to the basketball court, where the season is poised to start. The Mavericks, champions in 2011, are coming off a disappointing season in which they missed the playoffs.

Lauren D’Avolio contributed reporting.