Sen. Bernie Sanders, I-Vt., rallied supporters against the struggling Republican-led healthcare legislation making its way through Congress, saying in a speech Sunday the bill "must be defeated."

According to prepared remarks for a speech scheduled for Sunday afternoon in West Virginia, Sanders said the GOP bill as a threat to working-class families and a roll-back on access to health insurance.

"The so-called 'healthcare' bill passed in the House last month, strongly supported by President Trump, is the most anti-working class legislation ever passed in the modern history of our country and the Senate bill, also supported by Trump, in some respects is even worse," Sanders said in the speech, according to his prepared remarks.

The bill currently in the Senate is under fire from Democrats and centrist Republicans who say it doesn't do enough to expand healthcare coverage.

Echoing a theme from his failed run for the 2016 Democratic presidential nomination, Sanders said in his speech the U.S. should adopt a single-payer system.

"This bill is a moral outrage," he said. "It must be defeated. We should not be paying the highest prices in the world per capita for health care. We should not be paying the highest prices in the world for prescription drugs.

"We should have the guts to take on the insurance companies and the drug companies and move toward a Medicare-for-all, single-payer system."