Donald J. Trump may have lost his grip on Iowa, as the second poll in two days shows the billionaire Republican presidential candidate trailing Ben Carson in the state that kicks off the nominating process. A new Des Moines Register/Bloomberg Politics poll shows Mr. Carson with support of 28 percent of the likely participants in Iowa’s Republican caucuses, with Mr. Trump at 19 percent. In August, Mr. Trump was leading Mr. Carson, a retired neurosurgeon, by a margin of 23 to 18. The poll has a margin of error of five percentage points. Mr. Trump frequently parades his strong poll...