Donald Trump Jr. attacked the heritage of Rep. Beto O’Rourke (D-Texas) on Monday, tweeting that the Democratic challenger to the re-election of Sen. Ted Cruz (R-Texas) is “an Irish guy pretending to be Hispanic.”

“What’s authentic about an Irish guy pretending to be Hispanic? Asking for some friends Texas,” read a quote-tweet from the president’s eldest son attached to an ABC News article about Democrats’ craving for “authenticity” that spotlighted O’Rourke’s candor.

What’s authentic about an Irish guy pretending to be Hispanic? Asking for some friends Texas. https://t.co/c30OjHdN8j — Donald Trump Jr. (@DonaldJTrumpJr) October 15, 2018

Conservatives have long contested the use of the nickname Beto by O’Rourke, whose first name is actually Robert, because they claim it sounds Hispanic. Cruz has been at the forefront of those claims, running a radio ad earlier this year in which he says: “If you are going to run in Texas, you can’t be a liberal man. I remember reading stories, liberal Robert wanted to fit in. So he changed his name to Beto and hid it with a grin.”

O’Rourke, an El Paso native, has said Beto is a childhood nickname. He told Stephen Colbert on “The Late Show” in September that his mom stitched it on his sweater as 4-year-old.

Trump Jr. also went on a tear about Sen. Elizabeth Warren’s (D-Mass.) DNA test results that an expert said show “strong evidence’’ the senator has a Native American ancestor.

Trump Jr., who has retweeted dozens of disparaging remarks about Warren, tweeted Monday: “Amazing. As if having to bring in other genetics (read the actual article) and still only ending up with only 1/512th wasn’t enough of a joke.”