Leading politicians of Germany's left-wing parties on Tuesday called for the US ambassador to be expelled, after the staunch defender of Donald Trump was accused of meddling in domestic politics and aggravating already tense ties.
Richard Grenell had taken up his diplomatic posting on May 8, and immediately sparked an uproar when he tweeted on the same day that German companies should stop doing business with Iran as Trump quit the nuclear deal with the Islamic Republic.
He stoked further outrage over the weekend with his reported comments of his ambition to "empower other conservatives throughout Europe, other leaders".
Grenell also raised eyebrows with his plan to host Austria's conservative Chancellor Sebastian Kurz -- who the US envoy describes as a "rock star", for lunch on June 13.
"What this man is doing, is unheard of in international diplomacy," Martin Schulz, former chief of the Social Democratic Party, told national news agency DPA.
"If a German ambassador were to say in Washington that he is there to boost the Democrats, he would have been kicked out immediately.
"I hope that the Kurz visit leads to Mr Grenell's short tenure as ambassador in Germany," added Schulz, who also wrote on Twitter that the envoy did not behave like a diplomat but like a "far-right colonial officer".
Amid the row, Germany's foreign ministry is seeking clarification from Grenell about his comments, a topic that will be raised at a pre-arranged meeting scheduled for Wednesday between the envoy and state secretary Andreas Michaelis.
But the opposition far-left Linke party's chief Sahra Wagenknecht said Berlin would do better to immediately expel Grenell rather than invite him in for talks.
"Someone like US ambassador Richard Grenell, who thinks he can lord over Europe and determine who is governing here, he can no longer stay in Germany as a diplomat," Wagenknecht told Die Welt daily.
"If the government takes the democratic sovereignty of our country seriously, then it shouldn't just invite Grenell to a chat over coffee, but expel him immediately," she added.
The controversy surrounding Grenell comes as Germany and the United States are experiencing less than cosy ties after Trump ditched the Iran nuclear deal and imposed punishing tariffs on European aluminium and steel.
Underlining the souring transatlantic relationship, Chancellor Angela Merkel has warned that Europe can no longer rely on the US to protect it, but stressed that the continent must "take its destiny in its own hands".