By Sharon LaFraniere

Nearly 200 Democratic members of Congress are expected to file a federal lawsuit on Wednesday accusing President Trump of violating the Constitution by profiting from business dealings with foreign governments.

The plaintiffs — believed to be the most members of Congress to ever sue a sitting president — contend that Mr. Trump has ignored a constitutional clause that prohibits federal officials from accepting gifts, or emoluments, from foreign powers without congressional approval.

It is the third such lawsuit against Mr. Trump on the issue since he became president, part of a coordinated effort by the president’s critics to force him to reveal his business entanglements and either sell off his holdings or put them in a blind trust.

Like the previous two federal lawsuits, this one, to be filed in federal court in Washington, accuses Mr. Trump of illegally profiteering from his businesses in a variety of ways, including collecting payments from foreign diplomats who stay in his hotels and accepting trademark approvals from foreign governments for his company’s goods and services.

But it creates a new group of plaintiffs who claim the president’s actions have damaged them: Democratic members of the House and Senate who say they have been wrongly deprived of their constitutional right to rule on whether Mr. Trump can accept such economic benefits from foreign governments, according to Senator Richard Blumenthal of Connecticut, who led the effort with Representative John Conyers Jr. of Michigan.

“The founders ensured that federal officeholders would not decide for themselves whether particular emoluments were likely to compromise their own independence or lead them to put personal interest over national interest,” the lawsuit states. “An officeholder, in short, should not be the sole judge of his own integrity.”

Mr. Trump now faces three distinct groups of legal opponents, each alleging they have been harmed in a different way. Earlier this year, private individuals who own hotels or restaurants or book events at hotels that they say compete with Mr. Trump’s joined a lawsuit filed in federal court in New York by Citizens for Responsibility and Ethics in Washington, or CREW, a nonprofit watchdog group.

On Monday, the attorneys general of Maryland and the District of Columbia filed suit in federal court in Maryland, accusing Mr. Trump of putting hotels, resorts and convention centers owned or operated by their governments at a competitive disadvantage. Some legal experts have suggested that this suit may be the most likely to proceed because a state is considered a “coequal sovereign” of the president.

The president’s critics are clearly hoping to find a federal judge who will agree that the plaintiffs have enough standing to let a case proceed to the fact-finding stage. “It is pretty much copy and paste from one suit to another,” said Andy S. Grewal, a University of Iowa law professor who has been critical of the lawsuits’ merits.

He said the complaint from members of Congress was “the weirdest one” because a court ruling that would for the first time in 230 years define what constituted an emolument would also cover them. “They could be really shooting themselves in the foot,” he said.

Yet each new set of plaintiffs makes it harder for the Justice Department to defend the president on the grounds that his opponents have no legal standing to sue him, Mr. Trump’s critics said. “It puts the government in the position of saying that nobody can address this — not hotel competitors, not states, not members of Congress,” said Norman Eisen, the chairman of CREW, which started the legal efforts. “And you cannot get away with that in a rule-of-law system.”

Mr. Blumenthal, a former Connecticut attorney general, said the president’s companies did business in about 20 countries but were shrouded in secrecy, making it impossible for Congress to carry out its constitutional duty of determining whether he was receiving illegal benefits or emoluments. “The truth is we have no clue about the president’s investors,” he said in an interview with reporters Tuesday. “How much is Russian money?”

“What we are seeking first and foremost is disclosure,” he said. “We cannot consent to what we don’t know.”

In a response to the initial lawsuit, Justice Department lawyers argued that the framers of the Constitution never intended to prevent a president from owning a business or to ban ordinary, arms-length commercial transactions. They also contended that even if the president had violated the Constitution as his opponents allege, it is up to Congress to take action, not a federal court.

Mr. Trump’s defenders have said he has gone well beyond what is legally or ethically required to distance himself from his companies, which are now being run by his two adult sons.

Unlike the previous two lawsuits, the lawsuit by members of Congress does not accuse Mr. Trump of violating the so-called domestic emoluments clause that bans him from accepting benefits from states or the federal government. That is because the Constitution gives Congress no oversight role there.

A total of 196 members of Congress — nearly seven in 10 Senate Democrats and nearly nine in 10 House Democrats, but not a single Republican — joined the suit, Mr. Blumenthal said. He said members of Congress have previously sued presidents, alleging that they had been wrongly deprived of their right to carry out their constitutional duty.

But the number who joined the emoluments lawsuit is unprecedented, because the scope of the president’s business interests — and his alleged violations — “is truly unprecedented,” he said.