WASHINGTON — White House press secretary Sean Spicer apologized Tuesday after appearing to forget about the Holocaust while comparing Adolf Hitler with Syrian President Bashar Assad during a televised briefing with reporters.

His shaky performance and attempts to clarify the remark renewed criticism of Spicer, whose future in the role is a persistent source of speculation.

“You had someone as despicable as Hitler who didn’t even sink to using chemical weapons,” Spicer said, arguing that Russia and other countries that support Assad are on the wrong side of history.

Spicer’s remark ignored the horror of the Holocaust, in which gas chambers were used as part of a genocide that killed 6 million Jews as well as millions of others, including Roma and gay people.

An attempt by Spicer to clarify the statement only compounded the problem. “He was not using gas on his own people the same way,” Spicer said. He referred to the Syria bomb victims as “innocent.”

He then added that he was aware of “Holocaust centers” — an apparent fumbled reference to death camps — and that he meant that Hitler did not use gas in the middle of towns.

The suggestion that Holocaust victims were not Hitler’s “own people” — intended or not — hit a sore nerve for Jews and other victims who considered themselves loyal subjects of Germany. Hitler’s propaganda cast them as disloyal and inferior. There is also a history of Holocaust deniers who falsely claim, among other things, that gas chambers were not used to kill Jews.

After his own press briefing ended, Spicer attempted another clarification in written form that seemed to make a similar point. “In no way was I trying to lessen the horrendous nature of the Holocaust,” he said. “I was trying to draw a distinction of the tactic of using airplanes to drop chemical weapons on population centers. Any attack on innocent people is reprehensible and inexcusable.”

By Tuesday evening, Spicer was apologizing on CNN. He called his remarks a “mistake,” a “blunder” and an “insensitive and inappropriate reference.”

Democrats and Jewish groups condemned Spicer’s comments. House Democratic Leader Nancy Pelosi called for Spicer’s firing and demanded that Trump “immediately disavow his spokesman’s statements.”

Spicer’s comments came on the first day of Passover, in which Jews celebrate freedom from oppression, she noted. “While Jewish families across America celebrate Passover, the chief spokesman of this White House is downplaying the horror of the Holocaust,” Pelosi said.

The New York-based Anne Frank Center for Mutual Respect also called on Trump to fire Spicer.

The Associated Press contributed to this article.