Warren under fire, call for Trump impeachment at US Democratic presidential debate

She did not directly respond to questions about whether she would raise taxes for the healthcare plan, but she said she would not sign any bill that does not lower healthcare costs for middle-class families.

"I have made clear what my principles are here, and that is that costs will go up for the wealthy and for big corporations and, for hard-working middle-class families, costs will go down," she said.

The expansive Medicare for All proposal, based on the government-run healthcare plan for Americans over age 65, has sharply divided Democratic presidential contenders. Some analysts have said it would cost $32 trillion over a decade. Many other Democratic candidates back a Medicare-based plan as just one option for Americans seeking healthcare coverage.

US Senator Bernie Sanders, who sponsored a bill in the Senate to create a Medicare for All plan, said he thought it was "appropriate to acknowledge that taxes would go up" under the proposal.

Buttigieg plugged his plan for "Medicare for All Who Want It", a proposal that Warren poked fun at.

"Whenever someone hears the term Medicare for All Who Want It, understand what that really means is Medicare for All Who Can Afford It," Warren said.

US Senator Cory Booker tried to stay out of the fight, and warned the Democrats against tearing one another down during the debates. He urged them to keep their eyes on the goal of beating Trump.

"I have seen this script before," Booker said. "It didn't work in 2016 and it will be a disaster for us in 2020."