Former President Donald J. Trump on Tuesday doubled down on his widely condemned comment that undocumented immigrants are “poisoning the blood of our country,” rebuffing criticism that the language echoed Adolf Hitler by insisting that he had never read the Nazi dictator’s autobiographical manifesto.

Mr. Trump did not repeat the exact phrase, which has drawn criticism since he first uttered it in an interview with a right-leaning website and then repeated it at a rally in New Hampshire on Saturday.

But he said on Tuesday night in a speech in Iowa that undocumented immigrants from Africa, Asia and South America were “destroying the blood of our country,” before alluding to his previous comments.

“That’s what they’re doing. They’re destroying our country,” Mr. Trump continued. “They don’t like it when I said that. And I never read ‘Mein Kampf.’ They said, ‘Oh, Hitler said that.’”

He added that Hitler said it “in a much different way,” without making his meaning clear.

Undocumented immigrants, he added, “could be healthy. They could be very unhealthy. They could bring in disease that’s going to catch on in our country.” And he again said that they were “destroying the blood of our country” and “destroying the fabric of our country.”

Mr. Trump and his campaign have dismissed the comparisons between his remark and language used by Hitler using the words “poison” and “blood” to denigrate those who Hitler deemed a threat to the purity of the Aryan race.