A Texas Democrat hasn't won any of the almost 30 statewide offices since 1994, the country's longest losing streak.
Cruz had attempted to shame O'Rourke's language throughout the campaign, even releasing a 30-second mashup of the Democrat swearing at events while warning Texans that the lawmaker's rallies were not a "great place to bring the kids". It was seen as not only a referendum on the Trump presidency, but also issues of gender, race, and violence.
"This election wasn't about me, and it wasn't about Beto O'Rourke", Cruz said.
Trump took the unexpected step of travelling to reliably Republican Texas during crunch time, staging a Houston rally to energise his base for Cruz just two weeks before Election Day.
Cruz was backed by his 2016 presidential opponent President Donald Trump and other prominent Republicans.
The race was the most expensive in the U.S. Senate's history and the first to surpass $100 million in total fundraising.
In 2016, Cruz ran for president in a crowded GOP primary. "I think we have every indication, based on turnout so far, that we're going to have a very good day tomorrow". "They're energized", Cruz said Monday.
But Cruz's win was not immediately evident, as might have been expected in a normal year for an incumbent Republican senator in Texas. "They're filled with rage and hatred for the president".
No matter what happens, O'Rourke is now a national Democratic star, using donations from across the nation to smash Senate race fundraising records - including raking in more than $38 million in the three-month period from July through September.