“What this man is doing is unique in the history of diplomacy,” former leader of the Social Democrats (SPD), Martin Schulz, told DPA on Tuesday.
Instead of taking a neutral stance towards his host country, Grennell was acting like the emissary of a political movement, said Schulz, who led the SPD until earlier this year.
“If the German ambassador in Washington would say that he is there to strengthen the Democrats, he would be expelled straight away,” Schulz added.
Grenell caused anger in Germany after giving an interview to the far-right news site Breitbart in which he said he wanted to “strengthen other conservative movements in the whole of Europe.”
He has also invited Austria’s conservative chancellor Sebastian Kurz to dinner later this month. The Austrian Chancellor recently formed a coalition government with a far-right party and has been a vocal critic of Germany’s refugee policies.
In his interview with Breitbart, Grenell called Kurz “a rock star”, describing himself as “a big fan.”
But politicians in Germany have been less that impressed by the invite, a move seen as an attempt to meddle in European affairs
“I hope that the Kurz visit will lead to Mr Grenell only surviving for a short time in the role of ambassador to Germany,” Schulz said.
On Tuesday Sahra Wagenknecht, leader of the Die Linke, also called for Grenell to be ejected from the country.
“Whoever decides like US ambassador Grenell has done, that they can rule like a laird over Europe, can no longer remain ambassador to Germany,” she told Die Welt.
“If the government is serious about the sovereignty of our country they shouldn’t be inviting him to coffee but expelling him.”
Grenell is to meet officials at the German Foreign Ministry later this week for a pre-arranged meeting. But the ministry has made clear that it expects the ambassador to clarify his recent remarks.
The openly gay, but staunchly conservative ambassador already sparked outrage on his first day in the job, when he appeared to order German businesses to stop doing business with Iran.
Several politicians publicly warned him at the time not to interfere in German domestic affairs.
'Why is he ambassador?'
The storm over Grenell comes as Germany and the United States are experiencing strained 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”.
Although Merkel herself belongs to the conservative camp, commentators said it was clear that Grenell was not seeking to bolster the chancellor's party base, but rather its opponents from the far-right.
“It is bad enough if that's what he is thinking, let alone saying it,” said the daily Berliner Zeitung.
Media outside Germany also weighed in on the debate, with Washington Post columnist Anne Applebaum arguing that Grenell clearly intended to back “nativist, pro-Russia and anti-pluralist” forces in Europe.
Grenell certainly fulfilled his task if he was sent to Germany to destabilise Merkel's coalition or the Atlantic alliance, she said, “but if those are not his orders… then a different set of questions has to be asked”.
“Why is the US ambassador in Germany giving an interview to Breitbart? Why is he involving himself in partisan politics? For that matter, why is he an American ambassador at all?”
The Financial Times meanwhile urged Berlin to respond to Grenell's “undiplomatic behaviour” by holding its line on adhering to the Iran nuclear deal, responding firmly to US tariffs as well as stepping up to its defence responsibilities.
“This makes national and regional security sense, and would build the groundwork for a relationship with the US based on mutual respect and common interests.
“Such a relationship may not be possible while US embassies in Europe are led by Trump loyalists. But Germany and Europe have been left with little choice but to take the long view,” warned the business daily.