Beto's positions are not outside of the mainstream in Texas.
This is from the NYT/Siena poll done more recently than the Qpac poll for TX-7, which was +21 Romney/+1 Hillary. This has Beto +7 over Cruz.
Do you approve or disapprove of the job Donald Trump is doing as president?
APPROVE
DISAPP.
DON’T KNOW
Voters n = 491
42%
52%
6%
Would you prefer Republicans to retain control of the House of Representatives or would you prefer Democrats to take control?
REPS. KEEP HOUSE
DEMS. TAKE HOUSE
DON’T KNOW
Voters n = 491
48%
48%
4%
Do you favor abolishing ICE, the Immigration and Customs Enforcement Agency?
SUPPORT
OPPOSE
DON’T KNOW
Voters n = 491
28%
63%
9%
Do you support a bill that would reduce legal immigration and provide funds for a wall along the U.S.-Mexican border?
SUPPORT
OPPOSE
DON’T KNOW
Voters n = 491
39%
54%
6%
Do you support or oppose a federal ban on the sale of assault-style guns and high-capacity magazines?
SUPPORT
OPPOSE
DON’T KNOW
Voters n = 491
60%
35%
5%
Are iIllegal immigrants living in the United States more likely than American citizens to commit serious crimes?
AGREE
DISAGREE
DON’T KNOW
Voters n = 491
23%
68%
9%
Does it bother you to hear immigrants speak a foreign language in a public place?
AGREE
DISAGREE
DON’T KNOW
Voters n = 491
20%
73%
8%
Are there places in your area, say within a mile of where you live, where you're afraid to walk alone at night?
AGREE
DISAGREE
DON’T KNOW
Voters n = 491
38%
60%
2%
If the general election for United States Senate were being held today, would you vote for Ted Cruz or Beto O’Rourke?
BETO O’ROURKE
TED CRUZ
DON’T KNOW
Voters n = 491
51%
44%
5%
https://www.nytimes.com/interactive/2018/upshot/elections-poll-tx07-1.html