Bernie Sanders has claimed Republicans "have no idea how they are going to bring forth a substitute proposal."
The US Senate has voted in favour of drafting a bill to repeal the health insurance scheme known as Obamacare.
Under the Affordable Care Act - a signature programme of outgoing President Barack Obama - around 20 million previously uninsured Americans gained health coverage.
However, it has also faced severe opposition since its inception - including from Republicans, trade unions and small business groups. It remains a divisive programme among voters, and concerns remain over its affordability.
The US Senate resolution, which was passed by a 51-48 majority, will be voted on in the House of Representatives next week.
Republicans in both chambers have said that scrapping Obamacare is a top priority for them.
President-elect Donald Trump has called the scheme a "catastrophe" which had to be replaced "very, very quickly".
However, efforts to replace the scheme have stalled, with no clear plans yet put forward on a replacement.
In a press conference yesterday, President-elect Trump said: "As soon as our [health] secretary is approved, and gets into the office, we will be filing a plan [...] and the plan will be repeal and replace Obamacare."
Senator Bernie Sanders claimed Republicans "have no idea how they are going to bring forth a substitute proposal."
In a tweet, he suggested: "We're the only major country not to guarantee health care as a right. Why are we debating whether to take it away from 30 million people?"
In a heated speech, he also highlighted high medicine prices in the US, saying "the time has come for us to stand up to the drug companies".
Why do Americans pay so much more than Canadians for prescription drugs? Because the pharmaceutical industry has bought the US Congress. pic.twitter.com/Si8q8otWj5— Bernie Sanders (@SenSanders) January 12, 2017