Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE slammed Rep. Beto O'Rourke Beto O'RourkeJimmy Carter says his son smoked pot with Willie Nelson on White House roof O'Rourke endorses Kennedy for Senate: 'A champion for the values we're most proud of' 2020 Democrats do convention Zoom call MORE (D-Texas) on Monday, saying the Democratic challenger to Sen. Ted Cruz Rafael (Ted) Edward CruzCruz blocks amended resolution honoring Ginsburg over language about her dying wish Trump argues full Supreme Court needed to settle potential election disputes Press: Notorious RBG vs Notorious GOP MORE (R-Texas) is "pretending" to be Latino.

"What’s authentic about an Irish guy pretending to be Hispanic? Asking for some friends Texas," Trump Jr. tweeted, linking to an article from ABC News that contended O'Rourke has kept the Senate race close partly because of a desire among Democratic voters to see authenticity in candidates.

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

O'Rourke, whose full first name is Robert, has been mocked by some on the right for his decision to go by Beto, a name that some conservatives say he chose because it sounds Hispanic.

O'Rourke has said it's a childhood nickname that stuck.

Cruz ran an ad earlier this year mocking O'Rourke's nickname.

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"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," the ad says. "So he changed his name to Beto and hid it with a grin."

Cruz's full name is Rafael Edward Cruz.

When asked earlier this year about his own name change, Cruz told CNN about his Cuban heritage.

"You're absolutely right. My name is Rafael Edward Cruz," he said. "I am the son of my father Rafael Cruz, an immigrant from Cuba who came to Texas with nothing."

Cruz leads O'Rourke by about 7 points, according to a RealClearPolitics average of polls, just weeks before the Nov. 6 midterm elections.