GREENVILLE, S.C. — For the first time last week, a poll showed Ted Cruz pulling ahead of the Republican presidential pack in Iowa. For him and his ardent fans, it was a validation — and a sign of things to come. A second poll released Saturday confirmed the finding: Cruz is now a solid front-runner in the state that will kick off the voting in February. As in his improbable Senate win three years ago in Texas, Cruz has proved shrewd and methodical in his bid for the White House, tireless and disciplined on the stump, and adept at tapping...