The GOP field

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

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 New Jersey, June 30, 2015. REUTERS/Brendan McDermid

Rick Perry speaks at the Southern Republican Leadership Conference in Oklahoma City, May 21, 2015. REUTERS/Rick Wilking

Former Senator Rick Santorum speaks to the crowd after formally declaring his candidacy in Cabot, Pennsylvania, May 27, 2015. Daughter Elizabeth is at left. REUTERS/Aaron Josefczyk

Senator Ted Cruz 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. REUTERS/Chris Keane

Senator Lindsey Graham holds three month-old Elia Lionhood during a campaign stop at MaryAnn's Diner in Derry, New Hampshire, June 2, 2015. REUTERS/Brian Snyder

Jeb Bush takes questions at a town hall meeting in Reno, Nevada, May 13, 2015. REUTERS/James Glover

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

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

Senator Marco Rubio autographs a magazine after he announced his bid for the Republican nomination during a speech at Freedom Tower in Miami, April 13, 2015. REUTERS/Carlo Allegri

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

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

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

Mike Huckabee speaks during a "Roast & Ride" campaign event sat the Central Iowa Expo in Boone, Iowa, June 6, 2015. REUTERS/Dave Kaup

Senator Rand Paul walks to embrace his wife Kelley before formally announcing his candidacy for president during an event in Louisville, Kentucky, April 7, 2015. REUTERS/Aaron P. Bernstein