Andreas Nick, a prominent Christian Democratic Union (CDU) lawmaker and a member of the Foreign Affairs Committee in the German parliament, said that the US envoy needs to rethink his role in Germany or risk becoming a "highly ineffective" ambassador.
He added that the language Grenell used in the interview was “highly inappropriate” and reminiscent of what German extremist politicians used to say in the 1930s.
"If you translate 'awakening' and 'empowering' in this context, very ugly German language comes up, which resonates with the 1920s and 1930s," Nick said, advising Grenell to learn more about German history "and the sensitivities that result from that."
Andreas Nick said that it was up to Washington to decide who it appoints as its representative in Berlin, but added that if Grenell continues in the same vein he risks losing his reputation in the country.
"If he is not here to be a traditional ambassador, but a PR person for the alternative-right movement in Europe," Nick said, "we will have an issue."
Former Social Democrat leader and ex-European Parliament President Martin Schulz has been equally critical of Grenell’s language.
"What this man is doing is unheard of in international diplomacy," Schulz said in an interview with the German news agency DPA.
In an interview with ultra-conservative US news website Breitbart, Grenell said that “There are a lot of conservatives throughout Europe who have contacted me to say they are feeling there is a resurgence going on." He explained this by conservative politicians’ ability to speak up on sensitive issues like migration, taxes and the need to cut red tape.
Richard Grenell earlier raised eyebrows in the Bundestag by urging German firms working in Iran to leave the country amid Washington’s plans to withdraw from the 2015 nuclear pact with Tehran.
In Washington, meanwhile, State Department spokeswoman Heather Nauert stood by Grenell saying that US envoys "have a right to express their opinions."
"As an American, we believe in the right to free speech that other countries elsewhere around the world have the right to elect whoever the population chooses," Nauert said.
"Ambassadors have a right to express their opinion. They are representatives of the White House whether it is this administration or other administrations, and we hear them voicing their opinions. And there are sometimes opinions that people may or may not like. And there is the right to freedom of speech as well," Nauert told a briefing on Tuesday.
The spokeswoman stressed that the United States and Germany had a "very strong" relationship, adding that Washington is not supporting any political party.
"We support democracy, we support countries figuring out for themselves who they want to vote in for office," Nauert pointed out.