The lawsuit by the Citizens for Responsibility and Ethics in Washington (CREW) cites the Foreign Emoluments Clause, which prohibits federal officeholders from accepting any "present, emolument, office or title" from a foreign state. An emolument is any kind of payment made to a federal official, according to the suit.
The suit argues that the clause prohibits Trump's business empire from accepting anything of value from a foreign government, including payments at his Washington hotel, without congressional consent.
At a press conference earlier this month, Trump promised to turn hotel profits from foreign governments over to the United States Treasury. But the suit says that step in no way solves the constitutional violation.
"The Constitution provides for no such exception or remedy to this clause," said the suit.
Even if there were an exception, the plan would be insufficient because it has no enforcement mechanism and because it proposes to turn over only profits, not all money from foreign governments, the suit says.
The suit, filed in federal court in New York, seeks attorney fees but no damages. It asks that the federal court find that Trump is violating the Constitution through his business interests. The group also hopes to use the discovery process of the lawsuit to make Trump's tax return public, CREW board chairman Norman Eisen told CNN on Monday. CREW argues that the tax returns are the only way to see what benefit Trump has received from foreign governments.
At Trump's press conference last week, Sheri Dillon, a lawyer for Trump, dismissed concerns that accepting hotel business from foreign governments would violate the Foreign Emoluments Clause.
"No one would have thought when the Constitution was written that paying your hotel bill was an emolument," she said.
"Paying for a hotel room is not a gift or a present and it has nothing to do with an office. It's not an emolument," she added.
On Monday, her office declined comment on the pending lawsuit against the president.
The lawyers attached to the case include Eisen, an ethics lawyer for President Obama, and vice-chairman Richard Painter, the chief ethics lawyer in the George W. Bush administration.
CREW characterizes itself as a "nonprofit legal watchdog group dedicated to holding public officials accountable for their actions." Painter says this suit goes beyond politics.
"We're not for big government and government spending and the rest of it. Millions of conservatives are sick and tired of corruption, and there's nothing liberal or conservative about this," he said on CNN's "New Day."
Constitutional law scholars Erwin Chemerinsky, Laurence H. Tribe and Zephyr Teachout, and Deepak Gupta are also on the case.
But it's not clear that the suit will be able to proceed.
That's because, in order to bring a suit, a person or organization typically must demonstrate that they have been legally harmed by the defendant, a legal status known as "standing." A court can throw out a case if the person bringing the suit is simply upset by another person's action, but cannot demonstrate they have standing to sue. Constitutional law expert Michael McConnell, a law professor at Stanford University, said he doubts that CREW will be able to prove it has standing to bring this case.
"I don't think it's getting very far. The standing issue is not even close," he said.