Marist is a high quality pollster, however I wonder why Cornyn is doing as well as Trump here and Hegar is lagging. I was told by some people that Cornyn was a weak incumbent and would do worse than Trump, but that doesn't seem to be happening
Still Lean R
Cornyn, despite his conversion to the Religion of Trump, still comes off as a Generic R. Ted Cruz was a unique case of awful, which is why Beto was able to make it so close. Plus, it didn't hurt that the D primary in 2018 was virtually uncontested, allowing Beto to treat 2017-18 as an 18-month general election campaign.
Hegar has had to deal with a very crowded field...in fact, the Texan version of the 2020 Presidential crowd. She's had to campaign just to survive a primary, not affording her the Beto Privilege.