Republican presidential candidate Donald Trump poses with a ring given to him by a group of veterans during a campaign event on the campus of Drake University Thursday, Jan. 28, 2016, in Des Moines, Iowa. (AP Photo/Jae C. Hong)
Trump introduces his Republican presidential candidate rivals Mike Huckabee and Rick Santorum at his veteran's rally in Des Moines, Iowa, January 28, 2016. (Rick Wilking/Reuters)
Veterans wait in the crowd for the start of U.S. Republican presidential candidate Donald Trump's speech at a veterans rally in Des Moines, Iowa ,January 28, 2016. (Rick Wilking/Reuters)
U.S. Republican presidential candidate businessman Donald Trump talks to a supporter as he tries to depart from the venue after speaking at a veteran's rally in Des Moines, Iowa, January 28, 2016. (Aaron P. Bernstein/Reuters)
Republican presidential candidates Donald Trump, center, and Mike Huckabee, left, applaud while listening to Rick Santorum during a campaign event on the campus of Drake University Thursday, Jan. 28, 2016, in Des Moines, Iowa. (AP Photo/Jae C. Hong)
Republican presidential candidate Donald Trump speaks at a event at Drake University in Des Moines, Iowa, Thursday, Jan. 28, 2016. (AP Photo/Andrew Harnik)
Republican U.S. presidential candidate Donald Trump waves goodbye at the end of his "Rally to Benefit Veterans" in Des Moines, Iowa, January 28, 2016. (Rick Wilking/Reuters)
A woman, left, tries to cover the face of a protester during a campaign event for Republican presidential candidate Donald Trump on the campus of Drake University Thursday, Jan. 28, 2016, in Des Moines, Iowa. (AP Photo/Jae C. Hong)
Republican presidential candidate Donald Trump's pregnant daughter, Ivanka, waves to the crowd as her father (off camera) jokes about her possibly having her baby in Iowa, during Trump's veteran's rally in Des Moines, Iowa January 28, 2016. (Rick Wilking/Reuters)
Harley Stone, 16, left, wears a shirt to show his support for Republican presidential candidate Donald Trump, before the start of a campaign event on the campus of Drake University Thursday, Jan. 28, 2016, in Des Moines, Iowa. (AP Photo/Jae C. Hong)
Republican presidential candidate Donald Trump smiles while speaking at a rally at Drake University in Des Moines, Iowa, Thursday, Jan. 28, 2016. (AP Photo/Andrew Harnik)