Grenell does not behave like a diplomat, but like a far-right colonial officer,” Martin Schulz, a senior centre-left MP and former president of the European Parliament told the German Press Agency.

“What this man is doing is unique in the history of diplomacy. If the German ambassador in Washington would say that he is there to strengthen the Democrats, he would be expelled straight away.”

Ralf Stegner, the SPD’s chief in the state of Schleswig-Holstein, said an ambassador should be “a diplomatic representative of his country not an election fighter for right-wing parties”, while SPD MP Thorsten Schäfer-Gümbel added: “European citizens don’t need a Trump vassal to tell them who to vote for. A US ambassador who meddles in the democratic process to such a degree is simply misplaced.”

Richard Grenell at his appointment ceremony (Reuters)

After the comments began to cause controversy, Mr Grenell took to social media to double down: “I stand by my comments that we are experiencing an awakening from the silent majority – those who reject the elites & their bubble. Led by Trump”. But he added that “the idea that I’d endorse candidates/parties is ridiculous”.In the US it is common for ambassador position to be handed out by an incoming president to political allies, donors, and ideological fellow travellers. Mr Grenell’s appointment was delayed for months by the US Senate after he was named by Mr Trump last year.

The expulsion of Mr Grenell would represent another blow for Mr Trump, who has notably struggled to get many of his political appointments confirmed by the US Senate and also been hit by waves of people leaving the administration.