Story highlights Progressives are going on offense, mounting a new push for single-payer health insurance

Leading the way is Vermont Sen. Bernie Sanders

(CNN) Democrats eying the 2020 presidential contest could soon face a "Medicare-for-all" litmus test from the party's progressive base.

After last month's failure of President Donald Trump and congressional Republicans to repeal Obamacare, progressives are going on offense, mounting a new push for single-payer health insurance.

Leading the way is Vermont Sen. Bernie Sanders -- whose megaphone on the left is loud enough to force others vying to lead the party to weigh in, too.

At a Friday night Boston rally alongside another liberal firebrand -- Massachusetts Sen. Elizabeth Warren -- Sanders vowed to introduce a "Medicare-for-all" bill in Congress "within a month."

"If every major country on earth guarantees health care to all people and costs a fraction per capita of what we spend," Sanders said, "don't tell me that in the United States of America, we cannot do that."

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