Willie Nelson surprised and galvanized fans Saturday night (Sept. 29) with a rousing new political anthem he'd written in support of Texas Rep. Beto O'Rourke, titled "Vote 'Em Out."

Nelson, who has received mixed responses from his fanbase in regards to his backing of the Democrat Senate nominee, performed the song during a rally at Austin's Auditorium Shores. "The biggest gun we got/ Is called the ballot box/ If you don't like who's in there, vote 'em out," he sang to the audience. He pulled no punches lyrically, urging listeners to drive out the "bunch of clowns you voted in."

Listen to a snippet of "Vote 'Em Out" posted by a fan:

The rally attracted a staggering amount of fans, with local news estimating a number between 15,000 and 20,000 in the audience. Along with musical performances from Nelson, Leon Bridges, Carrie Rodriguez, Tameca Jones and Joe Ely, elected officials Austin Mayor Steve Adler and Representative Lloyd Doggett provided speeches.

O'Rourke —who is running against incumbent Republican Ted Cruz—himself gave a rousing stump speech, as well as joined Nelson on stage briefly to collaborate on Nelson's hit "On the Road Again."

Although Nelson has received flak from his conservative fans regarding his support, the singer has not backed down, stating simply that he doesn't care, and everyone is entitled to his or her own opinion.

"My wife Annie and I have met and spoken with Beto and we share his concern for the direction things are headed," Nelson says. "Beto embodies what is special about Texas, an energy and an integrity that is completely genuine."

His son, Lukas, has also spoken out passionately in defense of his father's political choice, saying he supports his dad's decision to back O'Rourke.

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