The former Democratic presidential candidate spoke out after an independent report found that the House Republican plan to replace the Affordable Care Act – known as Obamacare – would leave 24 million more people without healthcare by 2026.
“If this legislation is passed, and millions of people are thrown off of health insurance, not able to get to a doctor when they must, thousands of Americans will die,” the veteran Vermont senator said.
“That’s what this legislation is about and it must be defeated. And I hope there is enough sense among some of the Republicans to vote against it.”
The Congressional Budget Office report found that the measures would shave $337bn (£278bn) off the federal budget deficit by 2026, but would leave an extra 14 million Americans without insurance in the next year alone.
The non-partisan group of analysts and economists also found that by 2018, five million fewer people would be covered by Medicaid – the social health care program for families and individuals with limited resources.
Around 14 million fewer people would enroll in the Medicaid programme by 2026, it estimated.
Mr Trump has promised the plan – called the American Health Care Act – will provide “insurance for everybody” and his administration quickly denounced the report’s conclusions.
Secretary for Health and Human Services Tom Price suggested it gave an incomplete picture because it did not account for further regulatory measures he may take in the future.
At a White House meeting with people affected by the Obama healthcare law, Mr Trump predicted that rates for health insurance will go “down, down, down” if Congress passes the new plan.
Mr Trump said that prices for coverage would drop and promised that Americans will be able to pick the coverage plan and the doctor they want.
“We are not going to have one-size-fits-all,” Mr Trump said.
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But Democrats have seized on the report and condemned the Republican’s proposals.
“The CBO score shows just how empty the president’s promises, that everyone will be covered and costs will go down, have been,” said the party’s New York leader, Chuck Schumer.
“This should be a looming stop sign for the Republicans’ repeal effort.”
His colleague and the party’s National Committee chairman, Tom Perez, added: “Donald Trump’s ‘insurance for everybody’ pledge was a big fat lie.”