Hillary Clinton went aggressively after presidential rival Bernie Sanders right from the start of the Democratic debate Thursday, saying his idealistic policies are unachievable.

"I'm not making promises that I cannot keep," the party front-runner said at the end of her closing statement, before launching after into an attack on Sanders's platform and support for a single-payer healthcare policy.

ADVERTISEMENT

"The numbers just don't add up from what Senator Sanders has been proposing," Clinton told the New Hampshire audience at the debate, hosted by MSNBC just five days before the state's primary.

"That's why all of the independent experts, all of the editorial boards that vetted both of us have concluded that it is just not achievable."

Clinton then homed in on Sanders' single-payer health policy, echoing a controversial attack made by her daughter Chelsea, who suggested the Vermont senator would help Republicans undo the progress made by President Obama under the Affordable Care Act.

"Senator Sanders wants us to start all over again," Clinton said. "This was a major achievement of President Obama, of our country, it is helping people right now.

"I am not going to wait and have this plunge back into a contentious national debate that has very little chance of succeeding."

Sanders suggested the former first lady was bending the truth.

"I am on the Health Education Labor Committee. That committee wrote the Affordable Care Act," he said. "The idea that I would dismantle healthcare in America while we are waiting to pass a Medicare for all is just not accurate."