O'Rourke went first and noted that both he and Cruz have small children - and it's tough to leave them to do their jobs.
The hourlong debate at Southern Methodist University focused on domestic policy.
An ex-punk rocker who has attracted glowing attention from national liberal circles for his high-energy campaign and monster fundraising, O'Rourke has been mentioned as a possible 2020 Democratic vice-presidential candidate and even a 2020 White House hopeful.
On taxes, Supreme Court judges, the Second Amendment and other issues, Democratic Rep. Robert "Beto" O'Rourke and incumbent Republican Sen.
This morning, Cook Political Report - a top independent analyst - shifted its rating of the contest to "toss up," a stunning blow to Cruz and Republicans after decades of GOP dominance in Texas. Cruz called O'Rourke "out of step with Texas", and O'Rourke responded that "only one of us has been to each county in Texas". The state "needs a senator who will work with the president where we can and stand up to him where we must", O'Rourke said.
"That injustice. continues to persist today", O'Rourke said. "Imagine the cost", O'Rourke said.More news: Cleveland Browns Win A Game For First Time Since Obama Presidency
President Donald Trump has announced that he'll rally in Texas for his once bitter rival Cruz next month - previously unthinkable in a state where Republicans usually rack up landslide victories.
A truck sporting a billboard plastered with a negative tweet from Trump about Cruz during the presidential campaign was parked outside the debate hall hours before the two men were set to face off.
"I think Congressman O'Rourke is passionate, he is energetic", Cruz then told the crowd.
Mr. O'Rourke was measured in his criticism of Mr. Trump, although he did not dispute Mr. Cruz's point that Mr. O'Rourke is the only Democratic Senate candidate in these midterms who has vowed to vote to impeach Mr. Trump should the chance arise.
He slammed Cruz for supporting Trump's trade policies, which he said have negatively impacted the Texas economy.