Democratic presidential candidates, from left, former Virginia Sen. Jim Webb, Sen. Bernie Sanders of Vermont, Hillary Clinton, former Maryland Gov. Martin O'Malley, and former Rhode Island Gov. Lincoln Chafee take the stage before Tuesday night's debate. The Associated Press

Sen. Bernie Sanders of Vermont and Hillary Clinton, the front-runners in the Democratic presidential primary, took each other on directly for the first time in Tuesday night's debate.

Hillary Clinton, questioned about her shifting positions, said, "Like most human beings, I do absorb new information, I do look at what’s happening in the world."

Former Maryland Gov. Martin O'Malley, left, listens as former Rhode Island Gov. Lincoln Chafee speaks during debate. Both candidates came into the debate far behind in the polls.

Hillary Clinton went on the offensive against her challenger Bernie Sanders, after barely mentioning any of her Democratic rivals in six months of campaigning.

Former Virginia Sen. Jim Webb speaks during Tuesday night's debate.

Former Rhode Island Gov. Lincoln Chafee speaks during Tuesday night's debate.

Former Maryland Gov. Martin O'Malley was one of three second-tier candidates who had to make a big impression in Tuesday's presidential debate.

While Sanders and Clinton sparred over a range of issues, Sanders defended Clinton on the issue of her use of private email when she was secretary of state, saying, "The American people are sick and tired are hearing about your damn emails." Clinton reached over to shake his hand and said, “Thank you, Bernie.”