Senator Barack Obama (D-IL) (R) stands in front of Senator Hillary Clinton (D-NY) as they arrive for U.S. President George W. Bush's annual State of the Union speech to a joint session of Congress at the U.S. Capitol in Washington in this January 23, 2007 file photo. Hillary Clinton announced her second run for the presidency on April 12, 2015 starting her campaign as the Democrats' best hope of fending off a crowded field of lesser-known Republican rivals and retaining the White House. (Photo by Larry Downing/Reuters)

U.S. Senator Hillary Clinton (D-NY) places her hand over her heart during the National Anthem at the 30th annual Harkin Steak Fry in Indianola, Iowa, in this September 16, 2007 file photo. (Photo by /Joshua Lott/Reuters)

U.S. Democratic presidential candidate Senator Hillary Clinton (D-NY) hugs her daughter Chelsea (L) amongst supporters at her "Super Tuesday" primary election night rally in New York, in this February 5, 2008 file photo. (Photo by Jim Young/Reuters)

US Secretary of State Hillary Clinton (L) is joined by her husband former U.S. President Bill Clinton and daughter Chelsea Clinton as she is ceremonially sworn in at the State Department in Washington, in this February 2, 2009 file photo. (Photo by Jonathan Ernst/Reuters)

U.S. Secretary of State Hillary Clinton (L) reacts while reading a local English newspaper before conducting a web chat with Chinese internet users at the U.S. embassy in Beijing in this February 22, 2009 file photo. (Photo by Reuters/China Daily)

U.S. Senator Hillary Clinton (D-NY) leaves a news conference arm-in-arm with U.S. President-elect Barack Obama (R) after being announced as his choice for U.S. Secretary of State in Chicago in this December 1, 2008 file photo. (Photo by Jeff Haynes/Reuters)

U.S. Secretary of State Hillary Clinton (C) gestures with Libyan soldiers upon her departure from Tripoli in Libya, in this October 18, 2011 file photo. (Photo by Kevin Lamarque/Reuters)

A North Korean soldier (L) looks in through the window of the T2 building as U.S. Secretary of State Hillary Clinton and U.S. Defense Secretary Robert Gates (R) tour the Demilitarized Zone (DMZ) in Panmunjom, South Korea, in this July 21, 2010 file photo. (Photo by Cherie Cullen/Reuters)

U.S. Secretary of State Hillary Clinton (L) hugs Myanmar's pro-democracy leader Aung San Suu Kyi as they meet at Suu Kyi's house in Yangon, in this December 2, 2011 file photo. (Photo by Soe Zeya Tun/Reuters)

U.S. Secretary of State Hillary Clinton reads her notes during a Security Council meeting to discuss Peace and Security in the Middle East during the 67th United Nations General Assembly at the U.N. Headquarters in New York, in this September 26, 2012 file photo. (Photo by Keith Bedford/Reuters)

An audience member raises her hand to ask a question of U.S. Senator and Democratic presidential hopeful Hillary Rodham Clinton (D-NY) during a campaign stop in Keene, New Hampshire, in this February 11, 2007 file photo. (Photo by Brian Snyder/Reuters)

U.S. President Barack Obama (2nd L) and Vice President Joe Biden (L) and Secretary of State Hillary Clinton (seated, 2nd R), along with members of the national security team, receive an update on the mission against Osama bin Laden in the Situation Room of the White House, in this May 1, 2011 file photo. (Photo by White House/Reuters)

U.S. Secretary of State Hillary Clinton attends a news conference at the Great Hall of the People in Beijing in this September 5, 2012 file photo. (Photo by Feng Li/Reuters)

Former U.S. Secretary of State Hillary Clinton and her husband former U.S. President Bill Clinton hold up some steaks at the 37th Harkin Steak Fry in Indianola, Iowa, in this September 14, 2014 file photo. (Photo by Jim Young/Reuters)

A mobile phone shows a Facebook page of campaign propaganda to promote Hillary Clinton as president in 2016, in this photo illustration taken April 13, 2015. By one estimate U.S. online political advertising could quadruple to nearly $1 billion in the 2016 election, creating huge opportunities for digital strategy firms eager to capitalize on a shift from traditional mediums like television. (Photo by Mike Segar/Reuters)

An unknown artist placed a poster on a traffic signal in front of the building where Hillary Rodham Clinton's presidential campaign offices are located, Sunday, April 12, 2015 in the Brooklyn borough of New York. A top adviser to Clinton announced her much-awaited second campaign for the White House on Sunday in an email to alumni of her first presidential campaign. (Photo by Mark Lennihan/AP Photo)

Former Secretary of State Hillary Clinton talks with local residents as she campaigns at the Jones Street Java House in LeClaire, Iowa April 14, 2015. (Photo by Rick Wilking/Reuters)

A Hillary Clinton sticker is pictured on a street post near her campaign headquarters in the Brooklyn borough of New York, April 14, 2015. An unknown entity placed “Don’t say” signs outside Clinton's Brooklyn Heights headquarters before her announcement Sunday. (Photo by Brendan McDermid/Reuters)