WASHINGTON — President Trump ramped up pressure against the Justice Department on Friday, raising concerns about the possibility of a government informant inside his presidential campaign and saying “if true” it is the “all time biggest political scandal!”

In a pair of tweets on Friday morning, the president offered his latest salvo in a protracted battle between top federal law enforcement officials and Mr. Trump’s allies on Capitol Hill.

At the heart of the new twist in the fight is a confidential government informant who, according to current and former officials, met several times with two of Mr. Trump’s campaign aides.