At a massive outdoor concert attended by an estimated 50,000 Texans Saturday night, country music icon and unabashed progressive champion Willie Nelson teamed up with Rep. Beto O'Rourke (D-Texas)—who is challenging Republican Sen. Ted Cruz—to deliver a simple message just over a month before election day: "Vote 'em out."

"If you don't like who's in there, vote 'em out/That’s what election day is all about, and the biggest gun we’ve got is called the ballot box," was just one of the rousing lines from a new song Nelson wrote for the much-anticipated gathering in Austin, which provided a brief glimpse of the unique enthusiasm O'Rourke has sparked in his campaign against Cruz, whose lead in the polls has dwindled rapidly in recent weeks.

Some of @WillieNelson’s new “Vote Em’ Out” from the Beto rally: pic.twitter.com/uXQOUC4C56 — Andy Langer (@Andylanger) September 30, 2018

Donning a "Beto for Senate" t-shirt, Nelson—who has faced backlash from some of his right-wing fans over his support for O'Rourke—also performed the classic "On the Road Again" with some help from the Texas representative, a former musician in his own right.

Nelson's headline performance came just on the heels of a stump speech by O'Rourke that touched on a wide variety of topics, from the human-caused climate crisis to the Trump administration's cruel immigration policies.

"This is a campaign for the future, because the people of the future, our kids and our grandkids, are depending on what we do at this moment," O'Rourke declared at the fundraising event, which also featured hundreds of volunteers helping Texans register to vote ahead of the November 6 election.

"Let tonight be a message to the future," O'Rourke continued. "Let them know who we are, what we believe in and what we are willing to do to accomplish our goals. Let them know that we believe in this country. Let them know that we believe that we can come together and do great things for this country. And let them know that we believe that Texas can lead the way."

The crowd was pumped up tonight. Team Beto distributed posters to people near the front and there were frequent “Beto, Beto Beto” chants though the night. pic.twitter.com/TibbOsyGQC — Madlin Mekelburg (@madlinbmek) September 30, 2018 Omg y’all. Finally got cell service back after the @BetoORourke rally in Austin w Willie Nelson & Leon Bridges. Crowd estimate (w help from a few folks) was roughly 55,000. pic.twitter.com/dGUecAMgTG — Madlin Mekelburg (@madlinbmek) September 30, 2018

The enormous event came in the midst of an unexpectedly close race between O'Rourke and Cruz, who has in recent days resorted to launching bizarre attacks on his Democratic opponent—such as sharing a widely-praised speech by O'Rourke in which he denounced the police murder of an unarmed black man.

According to recent polling, Cruz is barely clinging to a slight lead over O'Rourke—and one recent survey found O'Rourke is leading by two points.

O'Rourke has also consistently outraised Cruz by a huge margin, despite not taking money from corporate PACs.

"You have made something absolutely extraordinary happen in Texas," O'Rourke told supporters in a recent Facebook video. "Extraordinary because we can't find many other Senate races that have raised this kind of money in a three-month period. We can't find one in Texas, maybe ever, that has done this. And to do it in this way, through small-dollar contributions—that's what democracy looks like."