Donald Trump is “decapitating the leadership of the intelligence community in the middle of a national crisis”, senior Democrat Adam Schiff has charged, after the president fired the inspector general of the US intelligence community late on Friday night.

News of the firing of Michael Atkinson came as the death toll from the coronavirus pandemic in the US passed 7,000 and the White House faced continuing criticism for its handling of the response.

Trump raged about Atkinson at Saturday’s White House coronavirus taskforce briefing, claiming someone should “sue the ass off” the whistleblower whose complaint Atkinson relayed to Congress, per his constitutional duty.

“It’s unconscionable,” Schiff, the chair of the House intelligence committee, told MSNBC. “And of course it sends a message throughout the federal government and in particular to other inspectors general that if they do their job as this professional did, and Michael Atkinson was a complete professional, they too may be fired by a vindictive president.”

Atkinson, a Trump appointee, determined that the whistleblower’s report was credible in alleging Trump abused his office by attempting to have Ukraine investigate his political rivals.

According to a justice department legal opinion, Atkinson expressed concern that Trump potentially exposed himself to “serious national security and counter-intelligence risks” when he pressed Ukraine’s president, Volodymyr Zelenskiy, to investigate the Democratic presidential hopeful Joe Biden and his son during a call on 25 July last year.

Late on Friday, Trump said in a letter to Congress that Atkinson no longer had his “fullest confidence” and would be removed in 30 days’ time.

“He’s settling scores,” Schiff said of Trump. “We’re in the middle of this pandemic and thousands of people are dying and he is retaliating against people who are on his enemies list and doing it in the dead of night.”

On Saturday morning, the House speaker, Nancy Pelosi, called Atkinson’s firing “a brazen act against a patriotic public servant who has honorably performed his duty”.

“This latest act of reprisal against the intelligence community threatens to have a chilling effect against all willing to speak truth to power,” Pelosi said.

Whistleblower Aid, a group which supports “individuals who, lawfully, report government and corporate law breaking”, said Trump was “implementing a direct assault on critical federal oversight mechanisms implemented in the wake of Watergate”, the 1970s scandal which brought down Richard Nixon.

“Now is the time for truth telling,” the statement said. “The lives of Americans depend on whether public servants … can carry out their duties without interference, fear or retaliation.”

At Saturday’s White House coronavirus briefing, Trump was asked about his move against Atkinson. In a rambling and furious answer, the president claimed not to know and never to have met an official he nonetheless insisted did “a terrible job” and was “a disgrace to inspector generals”.

“It was my decision,” to fire Atkinson, the president said, adding: “I have the absolute right to do so.”

The whistleblower, supposedly guaranteed anonymity under federal law, has been named by Republican politicians, rightwing media figures and Trump, who retweeted a message in which the official was named.

“Everyone knows who the whistleblower is,” Trump said on Saturday, adding: “Frankly someone ought to sue his ass off.”

The relaying of the whistleblower’s complaint led to Trump being impeached by the Democratic-held House, where Schiff steered proceedings. But the president was acquitted in February after a trial in the Republican Senate in which Schiff led a team of managers prosecuting the House’s case.

Schiff’s remark about Trump “decapitating” the intelligence community also took in the departure of acting director of national intelligence Joseph Maguire, who also backed the whistleblower’s action and was controversially replaced by a Trump loyalist, Richard Grenell.

The Axios website reported on Saturday that Trump is considering placing Steve Feinberg, a billionaire investor and political ally, in the Office of the Director of National Intelligence.

In the heated atmosphere of Trump’s Washington, language as forthright as that used by Schiff is common. For example, in January, as the impeachment trial began, CBS News reported that Republican senators had been warned that if they voted against the president, “your head will be on a pike”.

Only one Republican, the Utah senator and former presidential nominee Mitt Romney, voted to convict the president and remove him from office.

Elsewhere in Washington on Saturday, the top US federal watchdog promised to continue to conduct “aggressive” independent oversight of government agencies.

Michael Horowitz, chair of the Council of the Inspectors General on Integrity and Efficiency (CIGIE) and the inspector general at the Department of Justice, said in a statement Atkinson was known for his “integrity, professionalism, and commitment to the rule of law and independent oversight”.

US inspectors general, charged with independent oversight of federal agencies, were recently tasked with broad surveillance of the government response to the coronavirus outbreak, including the historic $2.3tn package to mitigate its economic impact.

“The inspector general community will continue to conduct aggressive, independent oversight of the agencies that we oversee,” said Horowitz.

“This includes CIGIE’s Pandemic Response Accountability Committee and its efforts on behalf of American taxpayers, families, businesses, patients and healthcare providers to ensure that over $2tn dollars in emergency federal spending is being used consistently with the law’s mandate.”

Democrats have expressed concerns about how the stimulus cash will be doled out.

“We’re not here to create a slush fund for Donald Trump and his family, or a slush fund for the treasury department to be able to hand out to their friends,” said Senator Elizabeth Warren of Massachusetts.