The network had originally invited both candidates to participate in a town hall, with each candidate appearing separately on the stage in individual segments. The Cruz campaign had accepted but later backed out, the network said. CNN said in response to the Cruz campaign request that it would convert the event into a debate if the O’Rourke campaign agreed to the change.
It was unclear if Mr. O’Rourke was going to agree. Although three debates were originally scheduled, one of them set for Houston was postponed, and it was uncertain whether it would be rescheduled. It seemed possible that Tuesday night’s debate was the final one.
For weeks, several polls showed the two candidates running about even, an extraordinary feat in Texas, where Democrats have failed to win any statewide offices since 1994. But lately the polls have shown Mr. Cruz building a lead over Mr. O’Rourke. One Quinnipiac University poll last week showed Mr. Cruz ahead by nine percentage points and another by The New York Times Upshot and Siena College had Mr. Cruz up by eight points.
Texas Democrats remain cautiously optimistic, with some more cautious and others more optimistic.
Few, if any, Democrats running statewide in Texas in recent decades have generated the amount of enthusiasm that Mr. O’Rourke has. In the 2014 race for governor, the party’s nominee, Wendy Davis, enjoyed an initial wave of excitement but then trailed the Republican, Greg Abbott, by up to 12 percentage points in the polls four months before the election, and some Democrats were publicly and privately grumbling about her campaign’s missteps.
Mr. O’Rourke is running a tighter race and a smoother campaign, effectively turning much of his life into a social-media live stream, allowing his supporters to watch him as he drives between events and gets a haircut.
“He’s probably running the best campaign that’s been run in Texas in my lifetime,” said Garry Mauro, a former state land commissioner who was the chairman of Hillary Clinton’s campaign in Texas in 2016. “I’m on his Facebook. At one time yesterday he had 13,000 people watching him drive in the car.”