Barack Obama has called for voters to protect his landmark healthcare reform in a video to mark ten years since it was passed, but is still holding out on endorsing a candidate in the Democratic primary race.

“With your help, it's the closest we’ve ever come to universal coverage in America,” he said of the Affordable Care Act, in a clip released Monday by the liberal advocacy group Protect Our Care. “There are people alive today because of what you did.”

“Republicans will keep trying both in Congress and in the courts to rip away the care that millions of Americans rely on,” he added.

It was a rare return to the spotlight for the former president, coming on the same day that voters in six states take part in primaries to choose the Democratic nominee to face Donald Trump in November.

But despite praising his administration’s crowning legislative achievement, Mr Obama has still yet to endorse his former vice president Joe Biden — who has frequently touted his role in passing the reforms, and has campaigned on building on them.

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Bernie Sanders, Mr Biden’s only rival remaining in the race to become the Democratic nominee, has argued for replacing the legislation known as “Obamacare” with Medicare for All — a “single-payer, national health insurance program to provide everyone in America with comprehensive health care coverage, free at the point of service.”

The former vice president, meanwhile, plans to “build on the Affordable Care Act by giving Americans more choice, reducing health care costs, and making our health care system less complex to navigate,” according to his campaign website.

And yet Mr Obama is said to be holding fire in endorsing a candidate in order to let voters have their say first.

“He’s a big believer in sort of letting the process play out. And that is why he’s chosen not to put his thumb on the scale,” a person familiar with the president’s thinking recently told Vanity Fair.