Former CIA chief John Brennan doubled down Sunday on his charge that Donald Trump has engaged in “treasonous” behaviour and called on Congress to block the US president’s attempts to strip other intelligence officials of their security clearances.

Brennan has received an outpouring of support from former top-ranking intelligence officers – but not much from Republican lawmakers – since Trump revoked his top secret security clearance last week in retaliation for what the president called “unfounded and outrageous allegations.”

Trump served notice that other top former and current officials – all Democrats – also were at risk of losing their clearances, and the White House has reportedly drawn up more revocation orders, raising the chilling spectre of an “enemies list.”

In an interview with NBC’s Meet the Press, Brennan reaffirmed his most inflammatory accusation against the president.

“I called his behaviour treasonous. I stand very much by that claim,” he said. “These are abnormal times . . . I have seen the signs blinking red on what Mr Trump has done and is doing.”

Brennan levelled the charge after Trump met with Russian President Vladimir Putin in Helsinki last month and cast doubt on the US intelligence community’s conclusion that Moscow meddled in the 2016 election to sway it in Trump’s favor.

Special Counsel Robert Mueller is investigating whether Trump’s campaign colluded with Russia and whether the president sought to obstruction justice.

Earlier this week, Brennan denounced as “hogwash” Trump’s repeated claim there was “no collusion” with Russia.

“This is the time for members of Congress to step up,” he said on NBC, referring to a move by Democrats in the Senate to constrain the president’s ability to revoke clearances without following the normal process.

Republican Senator Ron Johnson, chairman of the Homeland Security Committee, defended Trump’s actions towards Brennan, telling “Fox News Sunday” that the former CIA chief had “crossed a line” in his criticism of the president.

But he added: “I don’t want to see this become routine. I don’t want to see it be politicized.”

Meanwhile, Trump on Sunday angrily denounced the federal probe of Russia’s 2016 election meddling as “McCarthyism at its WORST!” insisting that he had authorised his White House counsel to testify to bring the matter to a close.

Trump’s Twitter storm was set off by a front-page report in the New York Times that said White House counsel Don McGahn had provided Special Counsel Robert Mueller with an unusually detailed account of Trump’s thinking during key episodes under investigation.

Trump – who had already addressed the issue late Saturday – stepped up his attacks on the story and the probe in a series of angry morning tweets, slamming The New York Times for implying that McGahn had turned on him.

“I have nothing to hide . . . and have demanded transparency so that this Rigged and Disgusting Witch Hunt can come to a close. So many lives have been ruined over nothing – McCarthyism at its WORST!”

Mueller is investigating whether the president sought to obstruct justice as well as whether his campaign colluded with Russia’s covert effort to sway the 2016 presidential election in Trump’s favour.

In more than 30 hours of testimony over the past nine months, the Times said McGahn laid out for investigators Trump’s fury over the probe and the ways in which he asked McGahn to respond to it.

Among the episodes he testified to were Trump’s firing of former FBI director James Comey, and his obsession with putting a loyalist in charge of the probe, according to the Times. It noted that McGahn played a key role in stopping Trump from firing Mueller, who was made special counsel after Comey’s firing.