People Willie Nelson

In this Jan. 7, 2017, file photo, Willie Nelson performs in Nashville, Tenn. Nelson has called out current immigration policies as “outrageous” and extended an offer to meet President Donald Trump at one of the detention centers at the U.S.-Mexico border.

On a muggy Fourth of July night just outside of Austin, Rep. Beto O'Rourke, D-Texas, walked on stage wearing a light blue button-down, with a Texas-sized American flag in the background.

But rather than one of his campaign rallies in his Senate bid against Republican Ted Cruz, the Democrat was in front of the thousands that came out for Willie Nelson's Fourth of July Picnic, an annual celebration of country music started by the legendary singer 45 years ago.

Met with a mix of wild cheers and a fair share of booing on the soggy holiday, O'Rourke explained why this Independence Day mattered even more to the country, expressing hope that "the big, bold, confident, strong people of the state of Texas" would help show the rest of the U.S. that there's no need to be afraid of immigrants, Muslims or anyone else whom might be considered different.

"It's my honor to be here tonight, to be able to work with so many of you," he said just before the fireworks, "to be with legends like Ray [Wylie Hubbard] and Willie, whom ensure that in times of disappointment and darkness that we meet that with power and joy and rock and roll music."

Soon thereafter, O'Rourke, whose punk rock roots would later be rehashed by the Cruz campaign, shed his button-down for a black T-shirt and guitar, and joined Nelson and the rest of his band on stage for spirited renditions of "Roll Me Up and Smoke Me When I Die" and "Will the Circle Be Unbroken" to close out the show.

For Nelson, the jam session with O'Rourke was the latest in a career filled with activism and liberal-leaning political stances that have largely gone against the norm for country music culture.

But for some of Nelson's fans, his partnership with O'Rourke is the breaking point. After Nelson announced on Wednesday that he would be headlining a Sept. 29 rally for O'Rourke in Austin, the first public concert he's ever held for a political candidate, fans have taken to social media to boycott and publicly disown the country legend.

On Nelson's Facebook page, where he posted an article from local outlet Austin360 with the announcement, some of the most striking remarks accuse the performer of aligning with, what they believe to be, the "socialist" positions of O'Rourke.

"Goodbye Willie," David R. Williams posted on Facebook. "I don't support socialist commies! You're not going to advertise on my FB page either." He added: "Like we say in Texas, Now Git."

"Open your eyes Willie!" Claudia Kirby Heathington wrote. "Beto is a Socialist whom probably has lied to you. This is a real shame you support him."

"[W]ow what a let down," said Melanie Philip. "You would pick a socialist agenda and an Anti American fellow like BETO, shame on you."

One man was so disgusted with Nelson that he's gone so far as to offer up his ticket to one of the legend's upcoming shows.

"I am no longer willing to watch that hippie guitarist whom supports that . . . socialist running for Senate," tweeted Dakota Bell on Wednesday.

Others are much more supportive of Nelson, and questioned those skeptical of the country legend.

"Not sure what went wrong in your life that would make you insult Willie Nelson," tweeted Wheeler Walker Jr., a country singer and songwriter, on Thursday. "You can argue politics all you want but you cannot argue Willie."

In a news release announcing the concert, Nelson offered insight as to why he's backing O'Rourke.

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

The outrage comes as O'Rourke and Cruz are entangled in a tight Senate race, described by the incumbent as "a dogfight," that has gained national spotlight in recent months.

Nelson's political activism goes back decades, with his support of the environment, same-sex marriage and, famously, marijuana legalization. In 1980, President Jimmy Carter invited him to perform at the White House. Nelson later said that during the visit he smoked, what he called, "a big fat Austin torpedo" on the roof of the White House. His support of the environment, same-sex marriage and, famously, marijuana legalization. He's supported former president Barack Obama, Sen. Bernie Sanders, I-Vt., and Hillary Clinton in recent years.

He's even talked about President Donald Trump, releasing a song last year inspired by the president called "Delete and Fast-Forward."

"Delete and fast-forward the news. The truth is the truth, but believe what you choose," Nelson sings. "When we blow the whole world back to where it began, just delete and fast-forward again."