Washington (AFP) - Outraged Democrats vowed to appeal a federal judge's ruling that could undo the US health care law known as Obamacare, saying on Saturday that they will use their new power in Congress to hold Republicans responsible and "expose their lies."

Related Video: Federal Judge Rules Obamacare Is Unconstitutional

US District Judge Reed O'Connor ruled late Friday that the health insurance reform, officially known as the Affordable Care Act (ACA), is unconstitutional.

The White House said it expects the ruling to be appealed to the Supreme Court. For now, it said in a statement, "the law remains in place."

But Democrats, who have seen the law survive scores of legal and legislative attacks, vowed to fight back, saying the health coverage of millions of Americans is at stake.

Senate Democratic leader Chuck Schumer tweeted that Republicans had pretended to care about those protected by Obamacare "while quietly trying to remove that support in the courts. Next year, we will force votes to expose their lies."

Democrats see the law as a signature achievement of former president Barack Obama, while Republicans dismiss it as governmental overreach. Donald Trump made abolishing and replacing the program a central pledge of his presidential campaign.

In a separate Obamacare case in 2012, five of the nine Supreme Court justices upheld the law. All five remain on the court.

Still, it remains unclear how they might rule in the new case. If the decision is upheld, it could significantly disrupt the US health care system.

In his ruling Friday, the Texas-based judge said that the full Obamacare program was unconstitutional because Congress, in its 2017 tax overhaul, eliminated a penalty for anyone lacking health insurance who failed to sign up for the program.

The 2012 case was over whether such a penalty was legal -- but now that it is gone, O'Connor said, the whole ACA should be stricken down because that provision is "the keystone" of the program.

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Trump expressed delight at the court's ruling on a complaint brought by several Republican attorneys general and two Republican governors.

"It's a great ruling for our country. We'll be able to get great health care," Trump told reporters. "It was a big, big victory by a highly respected judge."

"As I predicted all along, Obamacare has been struck down as an UNCONSTITUTIONAL disaster!" he tweeted the previous day.

Trump urged Republican Senate Majority Leader Mitch McConnell and the House of Representatives speaker-designate, Democrat Nancy Pelosi, to "pass a STRONG law that provides GREAT healthcare."

- 'Monstrous endgame' -

The court ruling came on the eve of the deadline for people to sign up for ACA coverage for 2019.

Angry Democrats blamed Republicans for what they see as a debacle that could leave millions of Americans without health care.

Republicans "know that they can't repeal the ACA in Congress. So they've continued their crusade through the courts," tweeted Democratic Senator Elizabeth Warren, a consumer-protection activist. "The ACA is still the law of the land."

Polls consistently show strong public support for the ACA guarantee of coverage regardless of pre-existing health conditions -- an issue Democrats used with great success in last month's midterm elections as they won control of the House of Representatives.

"#Republicans' legal crusade against the #AffordableCareAct is a political stunt, but a dangerous one that puts health coverage and vital health protections for millions of Americans at risk," Democratic Senator Sheldon Whitehouse tweeted Saturday.

Democratic Representative Joe Kennedy said "they found one judge to agree, and now Medicaid expansion could be gone, pre-existing condition protections could be wiped out, prices skyrocket and millions lose insurance. And they call that success."

O'Connor's ruling "exposes the monstrous endgame of Republicans' all-out assault on people with pre-existing conditions and Americans' access to affordable health care," Pelosi said in a statement.

While the court's "absurd ruling will be immediately appealed, Republicans are fully responsible for this cruel decision," she said.

She vowed that when Democrats take control of the House next month, lawmakers "will move swiftly to formally intervene in the appeals process" to uphold Obamacare.

Opposing the Republican lawsuit were 17 Democratic attorneys general led by Xavier Becerra of California.

They argue that the tax law changes do not mean that the whole Affordable Care Act becomes unconstitutional.

The Texas ruling "is an assault on 133 million Americans with pre-existing conditions" and on all who rely on Obamacare coverage, Becerra said in a statement. "Our fight to save Obamacare is far from over."