President Trump’s assertion that undocumented immigrants are  “poisoning the blood of our country” is a menacing marker. There is no American race, and no bloodline to contaminate. There are sensible border policies, and less sensible ones, but the whole point of America is that ideals, not blood, bind the country. The Founders knew well the attractions of blood and soil, and rejected them in the Declaration. The only oath is to the Constitution.

We understand that Mr. Trump is expressing a frustration felt by many Americans. The  border under President Biden is a disaster. Customs and Border Protection reports that fiscal 2023 saw 2.48 million encounters at the border, the highest annual total ever. September saw a monthly record. A poll this month from the Wall Street Journal disclosed that just 27 percent approve of Mr. Biden’s handling of this issue, with 50 percent “strongly” disapproving.

Mr. Trump is entitled to a bit of a victory lap. He raised immigration as an issue in his 2016 campaign for president when it was largely dormant in American political life. His promise to “build a wall” was roundly denounced. Now, Mr. Biden is building that wall and kicking aside  environmental roadblocks to get it done. Aid to Ukraine and Israel now hangs on whether he can buck his party in respect of a system he acknowledges is “broken.”

If Mr. Trump’s  policy shows a kind of rough insight, his rhetoric evinces a shocking lack of understanding of the lineaments of the American story. Here’s how Israel Zangwill, a British Jew, put it: “America is God’s Crucible, the great Melting-Pot where all the races of Europe are melting and reforming… Germans and Frenchmen, Irishmen and Englishmen, Jews and Russians — into the Crucible with you all! God is making the American.”

We understand that this idea — refined by Horace Kallen into a vision for a vibrant pluralism — is under attack from those who see an American as hopelessly tainted by sin and defined by racism and discrimination, for which the only response is more racism and more discrimination. Unfettered immigration may be handmaiden to human tragedy, from New York City to the Rio Grande. The free movement of labor is too important to be left to the left.

Human capital, though, is our greatest wealth, and Mr. Trump ought to know better than to talk of blood and poison. Winning immigration policy sees the immigrant as an asset. America has inspired pro-immigration policy on the right for decades. Mr. Trump’s leading challenger for the GOP nomination, Governor Haley, is herself descended from those who came here. As is Mr. Trump, whose wonderful wife is an immigrant herself.