By John Whitesides and Mike Stone

WASHINGTON (Reuters) – Democratic presidential front-runner Joe Biden unveiled a $750 billion healthcare plan on Monday that he said would strengthen the Affordable Care Act, drawing a contrast with rivals who back a more sweeping “Medicare for All” government-run system.

Biden portrayed White House rivals led by U.S. Senator Bernie Sanders of Vermont, who back a single-payer plan that eliminates private insurance, as a threat to former President Barack Obama’s signature healthcare law, known as Obamacare.

“I understand the appeal of Medicare for All. But folks supporting it should be clear that it means getting rid of Obamacare, and I’m not for that,” Biden, who was Obama’s vice president for eight years, said in a video.

Sanders is the lead Senate sponsor of a bill that would shift Americans into a Medicare-based, single-payer government-run system that largely eliminates private insurance. Medicare is the government insurance plan for the elderly.

Four other White House candidates in the U.S. Senate – Kirsten Gillibrand of New York, Cory Booker of New Jersey, Kamala Harris of California and Elizabeth Warren of Massachusetts – also support the proposal, which has become one of the main points of contention in the Democratic race to select a nominee to challenge Republican President Donald Trump in 2020.

Obamacare, a longtime target of Trump and other Republicans who have repeatedly tried to repeal it, is a popular program with Democratic voters.

Biden’s healthcare plan, estimated to cost $750 billion over 10 years and paid for partly by higher taxes on the wealthy, would include a public option that would let people enroll in a paid government healthcare plan that would exist alongside private insurance.

The former senator said he was “surprised” so many Democrats have proposed a shift to a Medicare for All plan, particularly since it would be hard to get through Congress and difficult to implement.

“It’s likely to be a bumpy ride getting to where they are going,” he said of Medicare for All supporters during an Iowa candidate forum sponsored by AARP, an advocacy group for older Americans. Iowa holds the first Democratic nominating contest of the 2020 election cycle.

Sanders, who has been running second in many opinion polls to Biden, responded in a statement that he had fought to pass and improve Obamacare and protect the Affordable Care Act from Republican efforts to repeal it.

“But I will not be deterred from moving forward to guarantee healthcare for all people,” he said. “At the end of the day, you’ve either got to be on the side of the people or the side of the health insurance companies. I know what side I’m on.”

U.S. Senator Michael Bennet of Colorado, another Democratic presidential contender who backs a Medicare-based public option, called on Sanders, Warren, Harris and other Democratic candidates to reconsider their support for Medicare for All.

“We must protect the Affordable Care Act,” Democratic U.S. Senator Amy Klobuchar of Minnesota said at the AARP forum. Klobuchar, another White House contender, also backs adding a public option to Obamacare.

Biden would pay for his healthcare plan partly by rolling back the Trump tax cuts for the wealthy and return the top rate to 39.6%, a senior Biden campaign official told reporters. The top income tax rate is currently 37 percent.

In addition, Biden’s plan calls for people earning over $1 million a year to pay double the tax they now pay on long-term gains made in the stock market. That rate is currently 20% for the highest earners.

Biden’s plan would increase the value of tax credits received by people who use the plan, a move designed to help them lower their premiums.

In addition, Biden would seek to repeal the existing law that prohibits Medicare from negotiating lower prices with drug companies.

(Reporting by John Whitesides and Mike Stone in Washington; Editing by Peter Cooney, Cynthia Osterman and Jonathan Oatis)