A protester carries a sign while holding an umbrella during a rally against President Trump on Tuesday. (Andres Kudacki/AP)

A federal judge on Friday gave the go-ahead to a lawsuit filed by 200 congressional Democrats against President Trump alleging that Trump has violated the Constitution by doing business with foreign governments while in office.

The lawsuit is based on the Constitution’s emoluments clause, which bars presidents from taking payments from foreign states without Congress’s consent. Trump’s business, which he still owns, has hosted foreign embassy events and visiting foreign officials at its downtown D.C. hotel.

Trump has not given Congress any details of these transactions, nor has he asked Congress’s permission for them. Trump says he doesn’t need to — by his reckoning, these transactions don’t fit the Founding Fathers’ definition of “emoluments.” They are business deals, he says, not payoffs.

But the Democratic members of Congress said Trump had effectively nullified their votes by not giving them anything to vote on.

In his ruling, Federal Judge Emmet G. Sullivan agreed with the legislators, writing that they have legal standing to sue and their case can proceed.

“The Clause requires the President to ask Congress before accepting a prohibited foreign emolument,” Sullivan wrote. If the allegations made by Democrats are true, he wrote, then “the President is accepting prohibited foreign emoluments without asking and without receiving a favorable reply from Congress.”

Typically, judges are skeptical when individual legislators seek to sue the president. In this case, Sullivan allowed it, writing that the lawmakers had no where else to turn for a remedy other than the courts.

The Department of Justice, which is representing Trump in this case, said in a statement that it will continue to fight the lawsuit.

“We believe this case should be dismissed,” said Kelly Laco, a spokeswoman for the department, “and we will continue to defend the President in court.”

The Trump Organization did not respond to a request for comment.

The decision opens another legal front for the president, who is facing an array of inquiries into his business, his campaign and his charity.

Trump is facing a separate emoluments suit filed by the attorneys general of Washington, D.C., and Maryland that is moving forward. He also is contending with the ongoing special counsel investigation into Russian interference, a lawsuit from the New York Attorney General that alleges “persistently illegal conduct” at Trump’s charitable foundation and a defamation lawsuit brought by former “Apprentice” contestant Summer Zervos.

Friday’s ruling is just the beginning of the Democrats’ lawsuit.

Sullivan still must rule on questions that include whether the Founding Fathers’ definition of “emolument” was broad enough to include a foreign embassy paying the president to rent a hotel ballroom.

Attorneys for the Democratic legislators called Friday’s ruling a significant victory.

“By recognizing that Members of Congress [have] standing to sue, the court proved to all in America today that no one is above the law, not even the President,” wrote Elizabeth Wydra, of the Constitutional Accountability Center, in a statement.

Although Trump has given up day-to-day management of his businesses, which include residential, office, hotel and golf properties in the United States, Europe and South America, he still owns them and can withdraw money from them at any time.

The foreign emoluments clause prohibits payments from foreign governments, which it says cannot be accepted “without the consent of Congress.”

Some of Trump’s properties benefit from investments or business from foreign governments, particularly his D.C. hotel, which has hosted leaders from Kuwait, Malaysia and other countries.

Neither Trump nor his company have said much about how much revenue comes in from such guests. In February, the company donated $151,470 to the U.S. Treasury, saying it was the “profits” from foreign government business in 2017. But it declined to explain how that figure was calculated.

The congressional plaintiffs, led by Sen. Richard Blumenthal (D-Conn.) and Rep. Jerrold Nadler (D-N.Y.), asked the court to force Trump to stop accepting payments they consider improper — or to force him to seek Congress’s consent first.

The argument over standing was, at its heart, a ruling about who had to go first — Congress or the president.

Trump’s contention was Congress ought to act first. If legislators believed Trump was violating the Constitution, he said, they could pass a law telling him to stop.

Congress’s contention was Trump had to act first and ask their permission. Sullivan agreed.

“Plaintiffs adequately allege that the President has completely nullified their votes,” Sullivan wrote, “because he has accepted foreign emoluments as though Congress had provided its consent.”

This is the latest case in which the president and his company may now be exposed to a lengthy legal process and possible discovery by plaintiffs who oppose him politically, a process that could include depositions of witnesses and the disclosure of Trump Organization financial documents.

In the other emoluments suit, the State of Maryland and the District of Columbia contend President Trump’s financial interests — particularly his D.C. hotel — allow him to unfairly profit. Justice Department lawyers in that case say the president is not breaking the law when foreign officials book rooms at his hotel in the capital because he is not trading favors in exchange for a benefit.

Last month, Justice Department attorneys in that case asked a federal judge to halt proceedings, arguing that allowing the case to go forward would “be a distraction to the President’s performance of his constitutional duties.” This sort of appeal is rarely granted, because lower court judges usually prefer to complete a full case before any appeal is made.

A third emoluments case, brought by the advocacy group Citizens for Responsibility and Ethics in Washington, was dismissed for lack of standing but is under appeal.

Before Trump took office, the emoluments clauses of the Constitution had not been tested in court in more than 200 years. One clause bars federal officers from taking gifts, or emoluments, from foreign governments. The other prohibits presidents from taking side payments from individual states.

Both aim to ensure independence and guard against undue influence by other governments.