Donald Trump said Comcast, the owner of NBC and MSNBC, “should be investigated for its ‘Country Threatening Treason’” and promised to do so should he be re-elected president next year.

In response, one progressive group said the former US president and current overwhelming frontrunner in the Republican 2024 presidential nomination race had “gone full fascist”.

The Biden White House said Trump threatened “an outrageous attack on our democracy and the rule of law”.

The US media was “almost all dishonest and corrupt”, Trump wrote on his Truth Social platform on Sunday, “but Comcast, with its one-side and vicious coverage by NBC News, and in particular MSNBC … should be investigated for its ‘Country Threatening Treason’.”

Listing familiar complaints about coverage of his presidency – during which he regularly threatened NBC, MSNBC and Comcast – Trump added: “I say up front, openly, and proudly, that when I win the presidency of the United States, they and others of the lamestream media will be thoroughly scrutinized for their knowingly dishonest and corrupt coverage of people, things, and events.”

Trump also used familiar terms of abuse for the press: “the enemy of the people” and “the fake news media”.

Observers reacted to Trump’s threat to NBC, MSNBC and Comcast with a mixture of familiarity and alarm.

In a statement, Andrew Bates, White House deputy press secretary, said: “President Biden swore an oath to uphold our constitution and protect American democracy. Freedom of the press is a fundamental constitutional right.

“To abuse presidential power and violate the constitutional rights of reporters would be an outrageous attack on our democracy and the rule of law. Presidents must always defend Americans’ freedoms – never trample on them for selfish, small and dangerous political purposes.”

Elsewhere, Paul Farhi, media reporter for the Washington Post, pointed to Trump’s symbiotic relationship with outlets he professes to hate, given that only last week Trump was “the featured interview guest last week on Meet the Press, the signature Sunday morning news program on … NBC”.

Others noted that on Monday night, the former White House aide Cassidy Hutchinson, a key witness for the House committee that investigated the January 6 attack on Congress, which Trump incited, was due to be interviewed on MSNBC.

“Female political or media antagonists really cause blood to come pouring out of Trump’s eyes,” wrote Howard Fineman, a columnist and commentator.

Sounding a louder alarm, Occupy Democrats, a progressive advocacy group, said Trump had gone “full fascist” with an “unhinged Sunday-night rant”.

“There you have it, folks,” it said. “While Trump and his Republican enablers love to falsely accuse Democrats of ‘weaponizing’ the government against Trump, Trump himself is now openly threaten[ing] to weaponize the presidency to completely remove entire news channels from the airwaves simply because they expose his rampant criminality.”

Juliette Kayyem, a Kennedy School professor and CNN national security analyst, pointed to a previous warning: “To view each of Trump’s calls to violence in isolation – ‘he attacked Milley’ or ‘he attacked NBC’ or ‘he attacked the jury, the prosecutor, the judge’ – is to miss his overall plan to ‘introduce violence as a natural extension of our democratic disagreement’.”

Trump’s rantings were also coupled with threats to Gen Mark Milley, the chair of the joint chiefs of staff whose attempts to cope with Trump were detailed in an Atlantic profile last week.

They come after a Washington Post poll gave Trump a 10-point lead over Joe Biden, who beat him in 2020, in a notional 2024 general election matchup.

The Post said the poll was an “outlier” but Trump dominates the Republican nomination race and generally polls close to Biden despite facing 91 criminal charges – for election subversion, retention of classified information and hush-money payments – and civil threats including a defamation trial arising from an allegation of rape a judge said was “substantially true”.

Another new poll, from NBC, showed Trump and Biden tied at 46% but Trump up 39%-36% if a third-party candidate was added. A “person familiar with White House discussions” about the prospect of a candidacy from No Labels, a centrist group, said it was “concerning”, NBC said. Biden, the report added, was “worried”.