Julian is a former San Antonio Mayor and was the Secretary of Housing and Urban Development under president Obama.
Jillson says Julian "is the more outgoing, gregarious of the Castro brothers. He would stand a better chance in a Texas senate race against Cornyn."
MJ Hegar, who run a close but unsuccessful race to unseat congressman John Carter in has declared and Jillson says state senator Royce West of Dallas is considering a run.
"Both of them would have to introduce themselves to the entire state. That is a process that takes a great deal of energy, a great deal of talent and a great deal of money. I think John Cornyn is sitting pretty today. If Julian Castro would get in to this race, that would give Texans something to talk about."