"When I raised concerns to Grenell about politicizing this post, he personally assured me that once he became ambassador he would stay out of politics," Murphy said on Twitter. "This interview is awful — ambassadors aren't supposed to 'empower' any political party overseas.”
Reinhard Bütikofer, a German member of the European Parliament, said Grenell would not be "very effective in his official function if he misunderstands his mission like that."
"Already his second major misstep," Bütikofer added in an apparent reference to the ambassador saying that German companies “doing business in Iran should wind down operations immediately" after Trump announced the U.S. was pulling out of the international nuclear deal with Iran.
“It’s totally out of line for a U.S. ambassador, particular to a friendly country, to make comments that associate himself with one lane of the political spectrum in the host nation,” said Stewart Patrick, a senior fellow at the Council on Foreign Relations, who focuses on international cooperation.
"This is a NATO ally and one of our closest partners in Europe and the last thing he should be doing is inserting himself in its political future," he added. "If the shoe was on the other foot, Americans would be appropriately outraged."
The president's son Donald Trump Jr., meanwhile, countered the criticism Sunday evening.
Grenell, a former U.S. spokesman at the United Nations under President George W. Bush, pushed back against his critics.
"The idea that I’d endorse candidates / parties is ridiculous," he said in a tweet. "I stand by my comments that we are experiencing an awakening from the silent majority — those who reject the elites and their bubble. Led by Trump."
In the same interview, Grenell also expressed admiration for Austrian Chancellor Sebastian Kurz, who won last year's election with a hard line on immigration and has formed a coalition with the far right Freedom Party.
"I think Sebastian Kurz is a rockstar. I’m a big fan,” he said.
A spokesperson for the German Foreign Ministry told journalists that officials had asked the U.S. for "clarification" and if Grenell's comments had been "given the way they were published."
Grenell received a master's degree in public administration from Harvard, was the longest-serving U.S. spokesman at the United Nations, and briefly worked for GOP presidential candidate Mitt Romney, according to the biography on his website.
He is also a founding partner of Capitol Media Partners, an international strategic communications firm with offices in Washington, New York and California.