Washington (CNN) Texas Sen. Ted Cruz's first shot at his expected Democratic challenger, Rep. Beto O'Rourke? Mocking his name.

As Tuesday's primaries were closing, Cruz's campaign released a 60-second radio ad that was a country music jingle prodding O'Rourke for going by "Beto" rather than "Robert."

"Liberal Robert wanted to fit in, so he changed his name to Beto and hid it with a grin," the song says.

O'Rourke said he's gone by "Beto" since he was born.

"My parents have called me Beto from day one, and it's just -- it's kind of a nickname for Robert in El Paso. It just stuck," he said in a brief phone interview Tuesday night.

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