On perhaps the defining issue of the 2016 Republican primary, Senator Ted Cruz falls well to the right of Ronald Reagan, who supported granting legal status to millions of undocumented immigrants. He opposes abortion even in cases of rape and incest, and has called for a federal amendment that would allow states to avoid performing or recognizing same-sex marriages. He wants to return to the gold standard, abolish the Internal Revenue Service and create a tax structure simple enough for Americans to file on postcards. He has criticized Donald J. Trump on deportation policy. From the right. Throughout his Senate...