Attorneys general of Maryland and the District of Columbia on Friday expanded their lawsuit accusing President Trump of accepting gifts from foreign and state governments, suing him not only as president but in his personal capacity as a businessman.

Legal experts say the move takes the "emoluments" clause of the Constitution into uncharted legal waters, since it has been interpreted as only applying to presidents in their official capacity.

"The conventional understanding is that once the president is sworn in ... everything he does is official, so he doesn't have a personal capacity any longer. That's kind of the assumption, but that could be wrong," said Mark Brown, a constitutional law expert at Capital University Law School in Columbus, Ohio.

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The lawsuit, filed in federal court in Maryland, is one of several recent cases challenging Trump's ties to his business ventures and his refusal to divest from them. The suits allege that foreign governments' use of Mr. Trump's hotels and other properties violates the Constitution's emoluments clause, which bans the president's acceptance of foreign gifts and money without Congress' permission. The clause has never been fully tested in federal court and Trump's Justice Department attorneys have argued that hotel room stays do not represent "foreign gifts."

Last month, during a five-hour hearing in a Maryland courthouse, U.S. District Judge Peter Messitte went round and round on the issue of official versus individual capacity, actually suggesting that the plaintiffs amend their lawsuit to include the president in his personal capacity.

"They're not talking about things he's doing as president, they're talking about something he's doing benefiting from as a private owner of a business," Messitte said in court, noting that the provision of hotel rooms has nothing to do with Mr. Trump's role as president. "Are you saying it does? ... Should he be sued in his official and private capacity?"

The move Friday to amend the lawsuit also potentially opens the doors to Mr. Trump's personal lawyers to join the case.

"While we continue to believe that the complaint would be sufficient as to the president in his official capacity, the court's questions at the hearing suggested that as an independent and alternate ground, it might be sound to proceed against the president in his personal capacity as well," said Norman Eisen, chairman of the watchdog group Citizens for Responsibility and Ethics in Washington, which is co-counsel on the case.

Judge Messitte has yet to make a decision on the government's motion to dismiss the case. If Messitte allows the case to proceed, it would likely move on to discovery. Some legal observers contend that getting to the discovery stage of the litigation, which would shed significant light on Mr. Trump's opaque and sprawling business empire, is the actual goal of the lawsuits.

Seth Tillman, a lecturer at Ireland's Maynooth University Department of Law who has written a brief in support of the government, criticized the plaintiffs for not suing the president in his individual capacity from the start and said that indicated the suit was mostly about getting new information through the discovery process.

"We've never seen litigation like this before," Tillman said, questioning why they didn't do this from the start.

Rob Marus, a spokesman for the Office of the Attorney General for the District of Columbia, said the case is about getting the president to stop violating the Constitution, not merely about getting to discovery.

According to Maryland Attorney General Brian Frosh: "This is the first time that anyone has had to sue a president for violating or nation's original anti-corruption law — the emoluments clause.

"During the hearing on the motion to dismiss, the judge's questions led us to the conclusion that it is advisable and perhaps necessary to sue Donald Trump as an individual," he told The Associated Press. "We took that step today."

The Associated Press emailed both the Trump Organization and the Justice Department for comment on the new motion. A Justice Department spokeswoman declined comment and the Trump Organization's spokesperson was not immediately available for comment

Late last year, a judge in New York threw out one such lawsuit filed by the watchdog group Citizens for Responsibility and Ethics in Washington, saying emoluments is an issue that Congress should address first. That case was appealed last week. A third federal lawsuit has been filed against Trump on the issue by nearly 200 Democratic members of Congress.

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Associated Press writer Stephen Braun contributed to this report.