In 2020, President Donald J. Trump gave a campaign speech in Minnesota railing against refugees and criticizing protests for racial justice. Toward the end, he wrapped up with standard lines from his stump speech and praise for the state’s pioneer lineage.

Then, Mr. Trump stopped to address his crowd of Minnesota supporters with an aside seeming to invoke a theory of genetic superiority. “You have good genes, you know that, right? You have good genes. A lot of it is about the genes, isn’t it, don’t you believe?” Mr.
Trump told the audience. “The racehorse theory, you think we’re so different? You have good genes in Minnesota.”

Mr. Trump’s mention of the racehorse theory — the idea adapted from horse breeding that good bloodlines produce superior offspring — reflected a focus on bloodlines and genetics that Mr. Trump has had for decades, and one that has received renewed attention and scrutiny in his third bid for president.

In recent months, Mr. Trump has drawn widespread criticism for asserting that undocumented immigrants are “poisoning the blood of our country,” a phrase that he said first in a right-wing media interview and has in the last week repeated on the campaign trail.