O'Rourke has built his campaign around healthcare, economic inequality, and resisting Trump and Cruz's immigration policies, as well as calling for some gun-control measures including uniform background checks and banning high-capacity weapons.
The debate at Dallas' Southern Methodist University also saw Cruz criticize O'Rourke's past support for legalizing marijuana, evoking his older half-sister, Miriam, who died of a drug overdose.
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.
"That's not true", Mr. O'Rourke rejoined, before expressing what he said were common sense moves Congress can make to ensure "weapons of war belong on the battlefield and not in public life". 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.
Sen. Ted Cruz slammed his opponent for the "extreme rhetoric" and defended the integrity of the vast majority of law enforcement officers.
The debate, slated to begin at 7 p.m. EST, marks the first direct face-off between the two men in a race that has vaulted to the top of the American political consciousness, as Democrats believe O'Rourke represents their best chance in decades to win statewide office in Texas for the first time since the early 1990's.
"I think Congressman O'Rourke is passionate, he is energetic", Cruz then told the crowd. Mentioning some lower-level judges Mr. O'Rourke said were unsavory yet earned Mr. Cruz's endorsement, Mr. O'Rourke contended Judge Kavanaugh's record with voting rights cases should give marginalized or minority populations in Texas pause.
"This goes to the heart of massive policy divisions", Mr. Cruz said, noting Mr. O'Rourke opposes Mr. Kavanaugh's nomination. President Trump has pledged to hold a rally in Texas with Cruz ahead of the election.
Such work would prove hard, Mr. Cruz said, given Mr. O'Rourke "has never talked to the president and wants to vote for impeachment". Cruz dismissed many such visits as photo-ops.