With 87 percent of precincts reporting, Cruz led O'Rourke by 51 percent to 48 percent and a margin of approximately 200,000 votes out of more than six million ballots cast.
"This was an election about hope and about the future, and the people of Texas rendered a verdict that we want a future with more jobs and more security and more freedom", Cruz told supporters after defeating O'Rourke.
Texas polls are set to close at 8 p.m. and 9 p.m. EST, per Politico. He raised a staggering $70 million, the most of any Senate candidate in history, according to USA Today.
The Democrat got the endorsement of Texan Willie Nelson, who performed at a rally.
Polling showed Cruz - vying for his second term - consistently ahead, but by single digits. "Besides the ability to regenerate his tail!"
Asian markets soar on hopes of end to US-China trade war
They also agreed to meet on the sidelines of the Group of 20 summit meeting in Buenos Aires, Argentina, at the end of the month. He gave no indication whether they made progress on settling an escalating tariff war over Beijing's technology policy.
"Texas came together behind a common-sense agenda of low taxes, low regulation and lots and lots of jobs", Cruz told supporters at an election night watch party in Houston. "He's dishonest", O'Rourke said then.
Seemingly unfazed by Triumph's reference to President Donald Trump's 2016 attacks on Cruz's wife, the Texas Senator reaffirmed his support for the Constitution and dismissed any concerns about Twitter. "It's why the nickname stuck - because it's true".
O'Rourke's campaign drew support from various celebrity figures, including singer Beyoncé, National Basketball Association star LeBron James and country music legend Willie Nelson.
"Really interesting to hear you talk about a partisan circus after your six years in the US Senate", O'Rourke shot back, alluding to Cruz's role in leading the Republicans in 2013 to shutting down the government.
Cruz, who has made no secret about wanting to run for president again once Trump leaves office, even gleefully tweeted national polls showing O'Rourke cracking the top 10 among potential Democratic White House hopefuls in 2020, arguing they show how liberal he is. So with Conan on hiatus this week, Stephen Colbert "borrowed" Triumph the Insult Comic Dog to send down to the lone star state to gauge the vibes at Beto and Cruz rallies. "Hey, I support spaying and neutering", he explained, "just like Trump did to you".