The comments from Grenell, a former spokesman for the U.S. at the United Nations during the Bush administration, have drawn some criticism over his apparent line-crossing when it comes to diplomatic norms.
"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. This interview is awful - Ambassadors aren't supposed to "empower" any political party overseas," Chris Murphy, a Democratic U.S. Senator for the state of Connecticut, said in a tweet.
The U.S. State Department and Germany's embassy in Washington, D.C., did not reply to emailed requests for comment sent outside of regular working hours.
Grenell's comments also came at a time when ties between the U.S. and Germany, as well as other members of the European Union, have become increasingly fraught. German Chancellor Angela Merkel, who former president Barack Obama once referred to as his "closest ally," has had a rocky relationship with U.S. President Donald Trump.
Some took the ambassador's comments as an affront to Merkel's government: She leads a center-right party that has been on the defensive against a growing far-right movement.
Tensions between the U.S. and the European Union have also run high, most recently after Trump's administration decided to apply tariffs on steel and aluminum imports on the European Union.