According to the Associated Press/Fox News exit polls, Beto won Latino men 63/35 (women 74/25). If Castro can improve that showing and hold Beto's suburban support, the seat is his.
Helps that Latino men faced ethnic/gender identity cross pressure from Cruz that Cornyn will not be able to exert.
He probably needs to not just hold Beto's suburban support, but to build a little bit off of it in order to actually win. As in
-clear 60% in Harris
-fight to a draw or near draw in Collin and Denton
-hold and build on the Stalinist margins out of metro Austin
IMO, these will come naturally just from the presidential race, considering he is solidly progressive and won't turn off any new voters in 2020. Castro's huge advantage is his ability to turn out Latinos, whose relatively low turnout compared to 2016 is likely what ended Beto. Combine high latino turnout and Beto(probably better actually, considering how fast these areas are growing) #s in suburbs and although Cornyn will gain a few 100k from high rural turnout, I would put it at pure tossup.
He will turn off moderates, Clinton won lots of moderates in TX, Cornyn is the inoffensive Generic R and a progressive like Castro will alienate moderates
In what world is Castro Progressive