NEW YORK — To fight what it called a “grave threat” to the country, a watchdog group Monday filed a lawsuit alleging that President Trump is violating the Constitution by allowing his business to accept payments from foreign governments.

The lawsuit claims that a constitutional clause prohibits Trump from receiving money from diplomats for stays at his hotels or foreign governments for leases of office space in his buildings.

The language in the clause is disputed by legal experts, and some think the lawsuit will fail. But it signals the start of a legal assault on what Trump critics see as unprecedented conflicts between his business and the presidency.

Trump called the lawsuit “without merit, totally without merit” after he signed some of his first executive actions Monday in the Oval Office.

The watchdog group Citizens for Responsibility and Ethics in Washington filed the lawsuit in the Southern District of New York.

The group is being represented in part by two former White House chief ethics lawyers: Norman Eisen, who advised Barack Obama, and Richard Painter, who worked under George W. Bush. The two said they feel they had no choice but to take legal action.