Democrats launched an investigation into the firing of the State Department's inspector general, its internal watchdog.
They also requested records pertaining to Inspector General Steve Linick’s removal to be submitted to Congress by Friday.
In announcing the probe, the Democrat chairman of the House Foreign Affairs Committee Eliot Engel and ranking Democratic senator on the Foreign Relations Committee Bob Menendez, asked White House chief of staff Mark Meadows to preserve and submit all the records.
They called the timing and motivation or Mr Linick’s firing an "unprecedented removal."
Mr Trump, in a letter to House Speaker Nancy Pelosi on Friday, said he no longer had confidence in Mr Linick's ability to serve.
The letter did not specify a reason for the latest in a string of government watchdogs to be removed in recent weeks under the Republican president.
Democrats have said Mr Linick had started an investigation into US Secretary of State Mike Pompeo.
"We unalterably oppose the politically-motivated firing of inspectors general and the President’s gutting of these critical positions," wrote House panel chairman Eliot Engel and Senator Bob Menendez, the ranking Democrat on the Republican-led Senate Foreign Relations panel.
They called on the Trump administration to turn over any related documents by May 22.
"This firing is the outrageous act of a president trying to protect one of his most loyal supporters, the secretary of state, from accountability," Mr Engel, a Democrat, said in a statement.
"I have learned that the Office of the Inspector General had opened an investigation into Secretary Pompeo. Mr Linick's firing amid such a probe strongly suggests that this is an unlawful act of retaliation,” he added without providing details on the scope of the investigation.
A Democratic aide told NPR that Mr Linick “was looking into the Secretary's misuse of a political appointee at the Department to perform personal tasks for himself and Mrs [Susan] Pompeo”.