Donald Trump Jr. made what critics saw as an apparent allusion to the Holocaust as he accused the news media of propping up Hillary Clinton during an interview Wednesday on WPHT Talk Radio 1210.

He suggested that if his father, or Republicans generally, had done the things Clinton did, the media would be preparing to gas them.

"The media has been her number-one surrogate in this," Trump Jr. said. "Without the media, this wouldn't even be a contest, but the media has built her up. They've let her slide on every indiscrepancy (sic), on every lie, on every DNC game trying to get Bernie Sanders out of the thing. If Republicans were doing that, they'd be warming up the gas chamber right now."

On Thursday, Trump Jr. told NBC News' Katy Tur the "gas chamber" crack was a reference to capital punishment, and not the Holocaust.

The gas chamber was first used in the U.S. in 1924, when Nevada introduced it as a more humane method of execution than hanging, according to the Death Penalty Information Center. Five states' laws authorize lethal gas for executions – Arizona, California, Missouri, Wyoming, and Oklahoma – though all designate lethal injection as the preferred method.

Arizona was the last state to execute a prisoner in the gas chamber - in 1999.

Despite Trump Jr.'s explanation, the remarks were roundly condemned by political opponents as anti-Semitic. Critics noted that both he and his father have retweeted messages from figures in the so-called "alt-right" white nationalist movement.

"The liberal, dishonest media is so quick to attack one of the Trumps that they never let the truth get in the way of a good smear," said Trump campaign spokesman Jason Miller. "Don Jr. was clearly referring to capital punishment to make the case that the media continues to take words out of context in order to serve as the propaganda arm of the Hillary Clinton campaign – something that's only gotten worse as Trump's poll numbers have improved."

The Anti-Defamation League tweeted its hope that Trump Jr. would retract his comment, saying that casual references to the Holocaust were never appropriate.

"It is horrifying that Donald Trump Jr. thinks it is appropriate to casually joke about the Holocaust - and even worse, he is defending it, saying it isn't anti-Semitic," Pennsylvania Democratic Chairman Marcel Groen said in a statement. "My grandparents died in Auschwitz, as did some of my aunts and uncles. They died in gas chambers. As a naturalized American citizen whose parents came to this country after World War II, I ask Trump to stop trying to reach out to the worst of us; the bigots and the anti-Semites. Making hatred mainstream is not an accomplishment; it is evil."

Trump Jr. made the reference duing an interview on The Chris Stigall Show. He was in Pennsylvania on Wednesday to open a campaign office south of Pittsburgh. (The remarks begin at about the 9:20 mark.)