Democratic presidential candidates Martin O'Malley, Hillary Clinton and Bernard Sanders stand together before the start of the Democratic presidential debate Sunday at the Gaillard Center in Charleston, S.C. (Associated Press)
Democratic presidential candidate, Sen. Bernie Sanders, I-Vt., speaks during a campaign stop, Monday, Jan. 4, 2016, in Manchester, N.H. (AP Photo/Mary Schwalm)
Sen. Bernard Sanders, vying with Mrs. Clinton for the Democratic nod, said that the ex-president's sexual life is a "nonissue" of the 2016 presidential campaign.
In a surprise to the Democratic Party, which thought Hillary Clinton would be the uncontested presidential nominee, Sen. Bernard Sanders drew voters in droves to rallies as he preached leadership from the left flank. (Associated Press/File)
Hillary Clinton, right, speaks to Bernie Sanders during a break at the Democratic presidential primary debate Saturday, Dec. 19, 2015, at Saint Anselm College in Manchester, N.H. (AP Photo/Jim Cole) ** FILE **
Bernie Sanders, left, offers an apology to Hillary Clinton during a Democratic presidential primary debate Saturday, Dec. 19, 2015, at Saint Anselm College in Manchester, N.H. (AP Photo/Jim Cole)
Democratic candidate Bernard Sanders is running a lean campaign. He has poured more than a quarter of his spending — some $3.1 million — into hats, T-shirts and bumper stickers to supply his enthusiastic fans. (Associated Press)
Tim Constantine reports on Hillary Clinton's take on the Benghazi committee, and a look at how she and Bernie Sanders are polling in Iowa and New Hampshire.
Democratic presidential candidate Bernard Sanders, who is gaining ground on front-runner Hillary Rodham Clinton, will be among the five White House hopefuls at the party's summer meeting this week in Minnesota.
Democratic presidential candidate Sen. Bernard Sanders is gaining more attention from potential voters in Iowa and New Hampshire due to his straight-shooting style and social agenda. (Associated Press)
Sen. Bernard Sanders has been cashing in on his quarter-century in the Senate as a fierce left-wing warrior in his bid to wean support away from presumed Democratic front-runner Hillary Rodham Clinton, who has been hamstrung by the ongoing scandal involving her private email server. (Associated Press)
Democratic presidential candidate Sen. Bernie Sanders, I-Vt., speaks at a rally, Monday, Aug. 10, 2015, at the Los Angeles Memorial Sports Arena in Los Angeles. (AP Photo/Ringo H.W. Chiu)
Democratic presidential candidate Sen. Bernie Sanders, I-Vt., speaks at a rally, Monday, Aug. 10, 2015, at the Los Angeles Memorial Sports Arena in Los Angeles. (AP Photo/Ringo H.W. Chiu)
Democratic presidential candidate Sen. Bernie Sanders, I-Vt., points to a campaign sign while being greeted by nurses during a visit to the National Nurses United office, Monday, Aug. 10, 2015, in Oakland, Calif. (AP Photo/Eric Risberg)
Martin Luther King III and Democratic presidential candidate Bernard Sanders commemorated the 50th anniversary of the Voting Rights Act but said more action is needed to make sure poor minorities in the U.S. are not disenfranchised. (Associated Press photographs)