Democratic candidates engaged in some of the sharpest exchanges to date in their last national debate before the presidential nomination voting process begins in Iowa on Feb 1.
Front runner Hillary Clinton put closest rival Bernie Sanders on the back foot over his newly announced healthcare plan and on gun control, while the Vermont senator accused the former secretary of state of taking too much money from the financial sector for speaking engagements and campaign contributions to impose tougher restrictions on Wall Street.
Former Maryland governor Martin O'Malley largely faded into the background in Sunday's debate.
"Front runners Hillary Clinton and Bernie Sanders did their best to contrast agendas and visions for moving the country forward following the presidency of Barack Obama," said Mr Aaron Kall, director of debate at the University of Michigan.
"Sanders talked about a revolution, while Clinton was much more pragmatic and would closely resemble a third term of President Obama," he said, adding that the "closely fought two-hour debate probably did little to move the needle".
Mr Sanders, more animated on Sunday than in previous debates, leads in the polls in New Hampshire and is closing in on Mrs Clinton's lead in Iowa.
Experts said that while there were no missteps from either of the top candidates, there was also no defining moment which might have tipped the scales. "Both had solid performances, but likely did little to influence the minds of undecided voters," added Mr Kall.
Mrs Clinton attacked Mr Sanders' healthcare plan, which he had released just hours before the start of the debate, saying it would set the country back in providing universal healthcare to all.
"We have accomplished so much already. I do not want to see the Republicans repeal it," she said, referring to the Affordable Care Act. "To tear it up and start over again... I think is the wrong direction."
But Mr Sanders defended his plan for a government-run health insurance programme, which would dump private health insurance premiums and help reduce large co-payments. "We're not going to tear up the Affordable Care Act. I helped write it. But we are going to move on top of that to a Medicaid-for-all system," he said.
He acknowledged that his plan would incur a tax on the average American, and Mrs Clinton pounced: "I want to raise incomes, not taxes."
On gun control, she said her rival had backed demands by the powerful National Rifle Association (NRA) gun lobby many times and criticised him for supporting legislation that gave gun manufacturers immunity.
Mr Sanders said he had voted for the immunity legislation to protect a "small mom-and-pop gun shop who sells a gun legally to someone".
He pushed on with his months-long assault on Mrs Clinton's Wall Street ties, trying to cast doubt on whether she would be beholden to large banks and corporations that are key fund-raising sources in the presidential process.
"I don't take money from big banks. I don't get personal speaking fees from Goldman Sachs," he said, vowing to break up the large financial institutions if he were to become president.
Mrs Clinton aligned herself with Mr Obama and his financial regulatory scheme. "I'm going to defend Dodd-Frank (Act)," she said.
Mr Sanders did not back down: "You do not go as far as reining in Wall Street as I would."
