SHANGHAI (AP) — Accepting a valuable trademark from the Chinese government without asking Congress first is unconstitutional, a group of 13 prominent Democratic senators said in a letter to President Donald Trump Thursday, warning that they intend to hold him accountable to his oath to uphold the U.S. Constitution.

"A president must not have two masters," the senators, led by Connecticut Democrat Richard Blumenthal, wrote. "If you continue to refuse to request and receive congressional approval before accepting favors from foreign governments, we will be unable to serve our constitutional role. Such a situation is unacceptable."

The letter comes in response to China's Feb. 14 registration of a trademark for construction services to Trump. He secured the mark only after fighting for 10 grinding years in China's courts to win back rights from a man named Dong Wei. A bureaucratic about-face after Trump declared his candidacy has raised questions about whether his political rise is benefiting his family business. These concerns are particularly sharp in China, where the courts and bureaucracy reflect the will of the ruling Communist Party.

Critics say the trademark award violates the emoluments clause of the U.S. Constitution, which bars public servants from accepting anything of value from foreign states unless approved by Congress. While the actual value of Trump's China trademarks is unclear, Trump himself has said he spent hundreds of thousands of dollars defending them. Trump has significant intellectual property interests abroad, including 49 pending and 77 registered trademarks in China alone. Most come up for renewal during his term.

Alan Garten, chief legal officer of The Trump Organization, has said that Trump's trademark activity in China predates his election. Trump has turned management of his company over to his children and a team of executives in order to remove himself from his business and its trademark portfolio, he added.

The letter comes after earlier criticism by Senator Dianne Feinstein, who called the China trademark registration "a clear conflict of interest and deeply troubling."

"If this isn't a violation of the Emoluments Clause, I don't know what is," the California Democrat said in a Feb. 17 statement. "These types of conflicts are exactly what we feared would happen: the president profiting off of his public service." Feinstein is also a signatory to this week's letter.

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