A former ambassador to Germany during the Trump administration and potential Republican challenger to Gov. Gavin Newsom in the event of a recall vote has found himself in hot water this week over a tweet comparing “vaccine passports” to the Nazi persecution of Jews.

Richard Grenell, a board member of the U.S. Holocaust Memorial Council in D.C., a staunch Trump ally and a frequent Newsom critic, assailed the idea of vaccine passports by tweeting out a photo on March 29 of actor Christoph Waltz playing a Nazi SS colonel in the 2009 film “Inglourious Basterds.”

“You’re hiding unvaccinated people under your floorboards aren’t you?” the photo says, with Grenell adding the accompanying text, “Speak up now. #slipperyslope.”

Speak up now. #slipperyslope pic.twitter.com/6WlDz0I9hQ

— Richard Grenell (@RichardGrenell) March 29, 2021

The reference is to a scene where Waltz, whose character is known as “The Jew Hunter,” discovers (and orders the execution of) a family of Jews and says to the farmer hiding them, “You’re sheltering enemies of the state, are you not?”

Some conservatives and libertarians say vaccine passports, which would allow those inoculated against Covid-19 to participate in certain activities, such as dining, working out at gyms and traveling, would violate civil liberties. While American nonprofits and companies are currently working on the passport idea, the White House recently said it would leave its implementation to the private sector. Israel has already implemented such a system, and the European Union and Japan are planning on creating one.

But the tweet from Grenell, a longtime conservative political adviser, diplomat and former acting director of national intelligence appointed by Donald Trump, drew condemnations from various Jewish groups.

The Anti-Defamation League called on Grenell to retract his tweet and pointed out that two years earlier he had written “Never compare the Holocaust to anything. Ever.”

Never compare the Holocaust to anything. Ever. https://t.co/gPPp8uDxxw

— Richard Grenell (@RichardGrenell) June 24, 2019

David Harris, CEO of the American Jewish Committee, tweeted at Grenell, saying he agreed with his 2019 tweet and adding, “Comparing anti-vaxxers to Nazi victims is so totally off-base, all the more so for ex-US envoy to Germany. Surely, other ways exist to make your point. Please reconsider.”

Grenell and the U.S. Holocaust Memorial Museum did not respond to a request for comment.

Nazi imagery and themes have been employed regularly by critics of Newsom and backers of the recall campaign. On March 24, California Legislative Jewish Caucus members defended the governor’s record and criticized the use of certain imagery, such as a photo of Newsom with a Hitler mustache. In November 2020, anti-maskers protesting his public health policies disrupted a Solano County supervisors’ meeting with Nazi salutes.

Grenell, who lists a residence in Riverside County, held the position of ambassador to Germany from May 2018 to June 2020. In December 2020, Trump appointed him to the 55-member board of the U.S. Holocaust Memorial Council, the governing body of the museum.

In a Feb. 27 speech at this year’s Conservative Political Action Conference, or CPAC, Grenell hinted he might consider a run against Newsom should there be a recall election. The recall effort criticizes Newsom’s approach to the pandemic, immigration and taxes, as well as his visit to a high-end restaurant in Napa County during California’s lockdown.

In an opinion article published in the Washington Post on March 30, columnist Paul Waldman argued that conservatives were “fearmongering” over the vaccine passport idea.

“The truth is that the privacy implications of vaccine passports are somewhat larger than zero, but way, way smaller than plenty of other intrusions on privacy that almost all of us — especially conservatives — accept every day,” Waldman wrote.