The former secretary of state scored a coup as she courts African-Americans, securing the endorsement of Rep. James Clyburn. The highest ranking African-American in Congress is an important power broker in South Carolina, where Clinton is trying to shore up a firewall against Sanders, who is giving her a much tougher race than anyone expected.
Sanders, meanwhile, accused his rival of pandering to African-American voters by cozying up to Barack Obama and even accused the President himself of being wrong on some issues.
He also noted he was once arrested in Chicago in the 1960s for fighting against racial segregation. And at a town hall event in Las Vegas, Sanders sought to lock in increasing support among female voters, saying he considered himself a committed feminist and had once been declared an "honorary woman" by feminist icon Gloria Steinem.
The exchanges reflect the need of both Democratic candidates to enlist the support of minorities ahead of the Nevada caucuses on Saturday and next week's first-in-the-south-primary. These are the first tests of a more diverse electorate than those seen in the first two contests in Iowa and New Hampshire.
Reaching out to Hispanic voters, Clinton released an emotive political ad showing her comforting a 10-year-old girl who confesses her fears that her parents could be deported. The former secretary of state beckons the girl to join her and tells her, "I'm going to do everything I can so you don't have to be scared."
"You don't have to worry about what happens to your mom and dad, or somebody else in your family," Clinton said in the ad airing ahead of Saturday's Democratic caucuses in Nevada. "Let me do the worrying. I'll do all the worrying, is that a deal?"
The spot offers a glimpse of an empathetic and emotional side of Clinton that she has sometimes struggled to put across on the campaign trail. It also underscores her strong support for President Barack Obama's controversial executive orders shielding some undocumented young people from deportation -- and her vow to go further if necessary.
Clyburn's endorsement represents an important morale boost for Clinton after her thumping loss to Sanders in last week's New Hampshire primary. Clyburn, a former civil rights activist, will unveil his endorsement for Clinton at 11 a.m. at Allen University in Columbia, the sources said.
Sanders suggested that Clinton had an ulterior motive in her embrace of Obama, notably in several Democratic debates.
"Everything the president does is wonderful. She loves the President, he loves her and all that stuff," Sanders said in an interview with CNN contributor Marc Lamont Hill, which will air on BET at 10 a.m. Sunday, according to excerpts from that interview provided by the network.
"And we know what that's about. That's trying to win support from the African-American community where the President is enormously popular," Sanders said.
The Vermont senator also took a swipe at Obama.
"I think he has done a great job in many respects," Sanders said. "But you know what? Like any other human being, he is wrong on certain issues."
At the Las Vegas town hall meeting, broadcast on MSNBC, Clinton took issue with Sanders over his criticism of Obama, earning cheers from her supporters but boos from her opponent's camp.
"He has called him weak, he has called him disappointing," Clinton said. "I don't know where all this comes from, maybe it's because Sen. Sanders wasn't even a Democrat until he decided to run for president."
But she also had a dramatic exchange with a Sanders supporter in the audience, who wanted to know why she didn't release transcripts of top dollar speeches she gave to financial institutions after she left the State Department. She said she wasn't the only candidate in the presidential race who had taken money for such appearances.
"I am happy to release everything I have when everybody also does the same," Clinton said.
Sanders, meanwhile, intensified his efforts to influence voters of color who will be pivotal to deciding the February 27 South Carolina primary. He held the latest in a series of meetings with African-American leaders, greeting senior figures in nine civil rights organizations in a meeting at the National Urban League headquarters in Washington.
And in the town hall meeting, Sanders also defended his criticism of some aspects of Obama's presidency. He had an uncomfortable moment when he was asked to listen to a clip of a radio interview in which he had suggested the president should face a primary in his re-election race in 2012.
He said that he considered Obama a friend, but it was natural for a sitting lawmaker to have differences with a president with whom he agreed on many other things.
"We have gone a long way together to move this country forward from the disastrous position we were in when Bush left office," Sanders said, saying he had worked with Obama for years.
Sanders used the talks with the civil rights leaders to condemn Senate Republicans for their refusal to hold a vote on Obama's forthcoming Supreme Court nominee, blasted what he said were GOP voter suppression efforts and decried economic conditions for African-Americans.
"It is absolutely incomprehensible that we have Republicans in the U.S. Senate - my colleagues in the Senate - who refuse to honor the Constitution of the United States," Sanders said at the meeting. "Correct me if I'm wrong, but the Constitution of the United States is pretty clear - that the President of the United States nominates individuals to the U.S. Supreme Court -- not a whole lot of ambiguity, is there?"
Sanders also said that African Americans had been more gravely affected than most communities from the recession that blitzed the U.S. economy in 2008.
"I understand the African-American community has been harder hit than any other community in America. I understand that it is unacceptable that 35% of black children in America are living in poverty," Sanders said.
Clinton and Sanders will continue their battle Thursday night at a Las Vegas town hall airing on MSNBC.
Clinton has a wide lead in South Carolina and is hoping to use it to stall the Vermont senator's momentum and raise questions about his capacity to translate a message that caught fire in less diverse Iowa and New Hampshire to a nationwide campaign.
A new CNN/ORC poll this week in Nevada showed Clinton with a single point lead over her rival, compared to a similar survey in October which had the former New York senator up by 16 points.
In South Carolina, however, Clinton's lead appears to be holding firm, so far. She was up 18 points in a CNN/ORC poll of South Carolina Democrats this week.