U.S. President Donald Trump demanded Sunday that the Justice Department investigate whether his presidential campaign was “infiltrated” or spied on for political purposes.

Trump has described the infiltration allegations as being potentially “bigger than Watergate,” but Democrats say he is attempting to undermine the year-old investigation into Russian interference in the 2016 election and possible collusion with his campaign, headed by special counsel Robert Mueller.

Trump’s angry order came amid building pressure from the year-old Russia meddling investigation, and as U.S. media reported that the Federal Bureau of Investigations (FBI) sent a Britain-based American professor to speak separately with Trump campaign advisers in 2016.

A New York Times report described the professor as “an informant,” saying the federal agency was looking into evidence that Carter Page and George Papadopoulos had suspicious contacts with Russia.

The Washington Post called him “a longtime U.S. intelligence source,” and said he met with a third campaign adviser — Sam Clovis — as well as Page and Papadopoulos. Trump and his supporters have cast the man as a mole possibly sent by the Obama administration to burrow into his campaign.

Efforts to identify the FBI informant have drawn fire from Democratic lawmakers.

“It would be at best irresponsible, and at worst potentially illegal, for members of Congress to use their positions to learn the identity of an FBI source for the purpose of undermining the ongoing investigation into Russian interference in our election,” said Senator Mark Warner, the Senate Intelligence Committee’s top Democrat. (SD-Agencies)