A senior Senate Democrat accused President Trump of "violating a key anti-corruption clause of the Constitution" by allowing foreign entities to rent space in his hotels.

Sen. Jeff Merkley, D-Ore., held a rally at an Oregon federal court two days after Democratic lawmakers filed a lawsuit in D.C. challenging Trump's right to benefit economically from business with foreign dealings. The lawmakers also note that "[t]he Chinese government [has granted] thirty-eight trademarks to the Trump Organization" since the president took office. It's the third lawsuit of its kind since January, a flurry of activity around "one of the most obscure and least-litigated clauses in the U.S. Constitution," as the legal industry publication put it.

"Trump is violating a key anti-corruption clause of the Constitution every day by failing to seek Congressional consent for his ongoing acceptance of gifts and payments from foreign governments," Merkley, who is rumored to be mulling a 2020 presidential campaign, said Friday. "Today, I joined my fellow Oregonians on the steps of the federal courthouse to send a message loud and clear: No one is above the law! If Trump won't comply with the Constitution and put the American people ahead of his personal profit, we will see him in court."

Merkley and his colleagues might not be allowed into court — or at least not very far — if they can't clear a procedural hurdle required in every lawsuit: the plaintiff has to have standing to sue, that is, the lawmakers have to convince a judge that they are harmed personally by Trump's activities for the case to proceed. "[A]ll three cases have serious standing problems," University of Iowa law professor Andy Grewal told ABA Journal, the American Bar Association's official publication.

A lawsuit filed in New York might have the best chance, Grewal suggested, because that suit includes a hotel and restaurant that can argue they're at a competitive disadvantage. The attorneys general for Maryland and D.C., who filed the remaining suit, have a difficult case to make. But they might benefit from the same kind of arguments that states have used to bring lawsuits against Trump's travel ban. "[S]ome courts (particularly the ones that have adjudicated the travel ban) have taken a broad view," the ABA Journal notes, citing Grewal.

Lawyers for the plaintiffs hope that the proliferation of the lawsuits will force a judge to take the case. "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 a left-leaning watchdog group called CREW, told The New York Times. "And you cannot get away with that in a rule-of-law system."