Suddenly, U.S. Sen. Ted Cruz, R-Texas, isn’t acting like a shoo-in for re-election. After his prime-time non-endorsement of GOP nominee Donald Trump at the Cleveland convention caused a firestorm, Cruz became the target of a lot of speculation. Already Democrats such as former State Sen. Wendy Davis, formerly of Fort Worth, and U.S. Rep. Joaquin Castro of San Antonio are talking about throwing their hats in the ring for U.S. Senate in 2018. As soon as Castro said in a joint CBS News’ interview with his brother, Secretary of Housing and Urban Development Julian Castro, that he was “going to...