Republican U.S. presidential candidate Ohio Governor John Kasich formally announces his campaign for the 2016 Republican presidential nomination during a kickoff rally in Columbus, Ohio July 21, 2015. REUTERS/Aaron P. Bernstein 

Wisconsin Governor Scott Walker, who is expected to make a bid to become a U.S. Republican presidential candidate, shows off his motorcycle driver's license before a "Roast & Ride" fund-raising event sponsored by Senator Joni Ernst (R-IA) in Des Moines, Iowa June 6, 2015. REUTERS/Dave Kaup 

Republican U.S. presidential candidate and New Jersey Governor Chris Christie shakes hands with supporters after formally announcing his campaign for the 2016 Republican presidential nomination during a kickoff rally at Livingston High School in Livingston, New Jersey, June 30, 2015. REUTERS/Brendan McDermid 

Rick Perry, former Texas governor and likely Republican presidential candidate, speaks at the Southern Republican Leadership Conference in Oklahoma City, Oklahoma May 21, 2015. REUTERS/Rick Wilking 

Republican presidential candidate and former U.S. Senator Rick Santorum speaks to the crowd after formally declaring his candidacy for the 2016 Republican presidential nomination during an announcement event in Cabot, Pennsylvania, May 27, 2015. Daughter Elizabeth is at left. REUTERS/Aaron Josefczyk 

U.S. Senator Ted Cruz (R-TX) stands on stage with his wife Heidi and their daughters Catherine and Caroline, after he confirmed his candidacy for president during a speech at Liberty College in Lynchburg, Virginia, March 23, 2015. Cruz, a conservative firebrand who frequently clashes with leaders of his Republican Party, became the first major figure from either party to jump into the 2016 presidential election race on Monday. REUTERS/Chris Keane

Republican presidential candidate U.S. Senator Lindsey Graham (R-SC) holds three month-old Elia Lionhood during a campaign stop at MaryAnn's Diner in Derry, New Hampshire June 2, 2015. REUTERS/Brian Snyder 

Former Florida Governor Jeb Bush takes questions at a town hall meeting in Reno, Nevada May 13, 2015. REUTERS/James Glover II TPX IMAGES OF THE DAY

U.S. Republican presidential candidate, real estate mogul and TV personality Donald Trump holds up his financial statement showing his net worth as he formally announces his campaign for the 2016 Republican presidential nomination during an event at Trump Tower in New York June 16, 2015. REUTERS/Brendan McDermid TPX IMAGES OF THE DAY 

U.S. Republican presidential candidate Carly Fiorina waits at her reception to greet her next attendee at the Republican Party of Iowa's Lincoln Dinner in Des Moines, Iowa, United States, May 16, 2015. REUTERS/Jim Young

U.S. Senator Marco Rubio (R-FL) autographs a magazine with his picture on the cover after he announced his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech at Freedom Tower in Miami, Florida April 13, 2015. REUTERS/Carlo Allegri TPX IMAGES OF THE DAY)

Republican presidential candidate and former New York Governor George Pataki greets supporters after formally announcing his candidacy for the 2016 Republican presidential nomination in Exeter, New Hampshire, May 28, 2015. REUTERS/Dominick Reuter TPX IMAGES OF THE DAY

Republican U.S. presidential candidate and retired neurosurgeon Ben Carson stands with his wife Candy as he officially launches his bid for the Republican presidential nomination in Detroit, Michigan May 4, 2015. REUTERS/Rebecca Cook

Republican presidential candidate and Louisiana Governor Bobby Jindal waves with his wife Supriya Jolly after formally announcing his campaign for the 2016 Republican presidential nomination in Kenner, Louisiana June 24, 2015. REUTERS/Jonathan Bachman (TPX IMAGES OF THE DAY)

U.S. Republican presidential candidate Mike Huckabee speaks during a "Roast & Ride" campaign event sponsored by Iowa Senator Joni Ernst (R-IA) at the Central Iowa Expo in Boone, Iowa June 6, 2015. REUTERS/Dave Kaup 

U.S. Senator Rand Paul (R-KY) walks to embrace his wife Kelley before formally announcing his candidacy for president during an event in Louisville, Kentucky, April 7, 2015. Earlier on Tuesday, Paul initially announced his candidacy in a post on his website. REUTERS/Aaron P. Bernstein