It also possible that O'Rourke won't be the only Democrat running.
Cruz's campaign has yet to comment.
Ted CruzTed CruzTexas Democrat to challenge Cruz: report The Hill's 12:30 Report THE MEMO: Frustrated Trump looks to turn it around MORE (R-Texas) in 2018, according to the Houston Chronicle. Here, O'Rourke take part in the 2016 President's Leadership Council. Joaquin Castro of San Antonio is considering a bid.
There is an additional potential outlier in this mix: Former George W. Bush strategist Matthew Dowd, an on-air commentator, is considering an independent run for Senate. He has toured the state in recent weeks, talking about his prospective Senate campaign in terms that made it sound like he has already reached a decision.
O'Rourke, who began his political career as a member of the El Paso City Council before being elected to Congress in 2012, is the polar opposite of Cruz.
Castro told CNN that O'Rourke called him Tuesday night to let him know he chose to run for Senate.
Castro has developed into a political firebrand for Democrats in the House since securing his seat for the 20th District of Texas in 2013.
No matter who the Democratic nominee is, a race against Cruz will be an uphill slog.
Cruz, meanwhile, appears to be steadying his political ship after a rough post-presidential campaign stretch that included being booed off the stage at the Republican National Convention. "We'll continue to work together where we can, but I don't think he's got a shot".
"It's pretty clear that running and losing a Senate race is a better option than circling the drain in the permanent Democratic minority in the House", Katie Martin, National Republican Senatorial Committee communications director, told CNN. O'Rourke has a knack for harnessing the Internet.
"Beto's been traveling across Texas for the past four months, meeting with people in communities big and small".