In the days before Christmas a dangerous new tone crept into the political combat surrounding the investigation into possible collusion between the Trump campaign and suspected Russian efforts to influence the outcome of the 2016 presidential election.

There have been warnings of a constitutional crisis and the reckless suggestion of a coup d'etat.

Special counsel Robert Mueller's investigation has moved with impressive pace. Credit:AP

Just over a week ago, high-profile Fox News host Jeanine Pirro, an old friend of Donald Trump, said on her program that there was a "criminal cabal" against Mr Trump inside senior federal law enforcement circles. She called for "a cleansing … in our FBI and Department of Justice ... it needs to be cleansed of individuals who should not just be fired, but who need to be taken out in cuffs".

The same day, her colleague Jesse Watters went further, warning his viewers of a potential coup. "The scary part is we may now have proof the investigation was weaponised to destroy [Mr Trump's] presidency for partisan political purposes and to disenfranchise millions of American voters," he said. "Now, if that's true, we have a coup on our hands in America."