Sen. Elizabeth Warren pledged Friday not to raise middle class taxes to fund her "Medicare for All" plan, responding to pressure she faced as she emerged as one of the frontrunners for the 2020 Democratic presidential nomination.

In a new outline, Warren's campaign said her single-payer health care plan would cost "just under" $52 trillion over a decade. It estimates the proposal would cost just less than the estimated $52 trillion for the current system.

Warren's campaign said her plan would give every American "full health coverage, and coverage for long-term care." It added that it would do so with "not one penny in middle-class tax increases." 

As she saw her support swell in both national and early state polls in recent weeks, the 2020 presidential candidate faced more pressure about how she would fund a single-payer, government run health care system. While she had not yet answered specifically whether her plan would hike taxes on middle class Americans, she has repeatedly argued it would cut costs by reducing spending on health care. 

"I will not sign a bill into law that does not lower costs for middle class families," she said at the October Democratic debate.

Sen. Bernie Sanders, a Vermont independent and the leading proponent of Medicare for All in the Democratic primary race, indicated to CNBC that he would not release a specific method to pay for his health care plan.

This story is developing. Please check back for updates.