Congressional Democrats on Saturday launched a probe into President Donald Trump's "politically-motivated" dismissal of a government watchdog believed to have been investigating Secretary of State Mike Pompeo.

The announcement came after Trump told House of Representatives Speaker Nancy Pelosi late Friday that he planned to dismiss State Department Inspector General Steve Linick.

It was Trump's third abrupt dismissal of an official tasked with monitoring governmental misconduct and abuse since April.

"We unalterably oppose the politically-motivated firing of inspectors general and the President's gutting of these critical positions," two senior Democrats -- Senator Bob Menendez and congressman Eliot Engel -- said in a statement.

The lawmakers said Linick had apparently "opened an investigation into wrongdoing by Secretary Pompeo himself." 

The IG's dismissal, "transparently designed to protect Secretary Pompeo from personal accountability, would undermine the foundation of our democratic institutions and may be an illegal act of retaliation."

A Democratic congressional aide, speaking on condition of anonymity, said Linick was probing complaints that Pompeo inappropriately used a political appointee to perform personal tasks for himself and his wife Susan.

CNN, citing a senior State Department official, reported that Pompeo himself had recommended the firing and hand-picked Stephen Akard, a former aide to Vice President Mike Pence, to succeed Linick. 

By law, the administration must give Congress 30 days' notice of its plans to terminate an inspector general, in theory giving lawmakers time to study the move -- and protest if warranted.

But previous such firings have gone through unimpeded, and those dismissed have been replaced by political allies of the Republican president.

Engel, who heads the House Foreign Affairs Committee, and Menendez, the top Democrat on the Senate Foreign Relations Committee, requested that the White House and State Department turn over records related to Linick's dismissal.

They also asked to see files for IG investigations "involving the Office of the Secretary that were open, pending, or incomplete at the time of Mr. Linick's firing."

Dog-sitting and takeout 

Pompeo has raised eyebrows for frequently traveling the world on his government plane with his wife, who has no official role.

CNN reported last year that a whistleblower had complained that the Bureau of Diplomatic Security, which guards US missions overseas as well as the secretary of state, had been assigned questionable tasks for the Pompeos, such as picking up takeout food or tending to the family dog.

The State Department confirmed Linick's firing but did not comment on the reason -- or on whether Pompeo was under investigation.

A State Department spokesperson also confirmed that the new inspector general would be Akard, an attorney who served as a foreign affairs advisor to Pence when the latter was governor of Indiana.

Since last year, Akard has led the State Department's Office of Foreign Missions, which handles relations with diplomats in the United States.

Pelosi said Linick was "punished for honorably performing his duty to protect the Constitution and our national security."

"The president must cease his pattern of reprisal and retaliation against the public servants who are working to keep Americans safe, particularly during this time of global emergency," she said.

Trusted presence for Trump

Pompeo is one of Trump's most trusted aides -- and a rare one never to come publicly into the crosshairs of the mercurial president.

In recent months, Pompeo has moved US foreign policy forcefully to the right -- encouraging a drone strike that killed a top Iranian general and promoting a theory, discounted by mainstream scientists, that the COVID-19 pandemic originated in a Chinese laboratory.

Linick, a longtime prosecutor, was appointed in 2013 by Trump's predecessor Barack Obama to oversee the $70 billion juggernaut of US diplomacy.

He played a small role in Trump's impeachment saga last year, handing to Congress documents by Trump's lawyer Rudy Giuliani with unproven claims about Democratic presidential candidate Joe Biden and Marie Yovanovitch, whom Trump removed as the US ambassador to Ukraine.

Trump repeated the charges to Ukraine's president and pressed him to dig up dirt, while freezing military aid to Kiev, which is battling Russian-backed separatists.

Since his acquittal by the Senate, Trump has fumed against a "Deep State" he sees as out to get him.