“The assumption has been for the last 20, 25 years, that Texas is solidly Republican,” he told NewsNation’s Leland Vittert. “But the demographics have been changing.”

He said the state is becoming less white and Democrats are getting better organized. In addition, Democrats this year are seeing “incredibly high” turnout in special elections and primaries around the country.

Texas Democratic Senate candidate James Talarico, who's currently a representative in the Texas House, speaks Wednesday for the first time since winning the Democratic nomination.

Eric Gay/AP

“If you’re a Democrat, you can’t wait to vote,” Luntz said. “Every Democrat is looking to participate.

Some Republican voters, on the other hand, are staying home.

And in Texas, there’s another issue: a vicious primary that’s about to get even uglier as the contest between state Attorney General Ken Paxton and incumbent Sen. John Cornyn is heading to a runoff.

But Paxton has indicated he will not.

Texas Attorney General Ken Paxton (left) is set to face Sen. John Cornyn in a runoff for the Republican primary for Cornyn's seat.

Julio Cortez/AP; Jack Myervia/AP

“The Republicans still have a clear advantage,” Luntz said. “But that advantage has been shrinking and shrinking. And when you have two candidates destroying each other, like these two are, this is the exact ingredient that you could have low Republican turnout in November, a split party, you could conceivably see a Democrat surprising people.”

Luntz also warned Republicans that control over the Senate could hinge on a single issue ― and it’s one that’s not helping the GOP at the moment: affordability.

“That’s it,” he said. “That’s the key decider.”

See more of that conversation below.