(Reuters) - Top congressional Democrats on Tuesday attacked a legal move by the Trump administration to step up its assault on the Obamacare healthcare law, saying last November’s elections showed Americans want them to protect the Affordable Care Act.

The Justice Department, in a two-sentence letter to the U.S. 5th Circuit Court of Appeals filed on Monday, said it backed a federal judge’s ruling in December that the Affordable Care Act violated the U.S. Constitution because it required people to buy health insurance.

The letter said the Justice Department would file a more extensive legal briefing later.

Democrats said the move to overturn Obamacare would overshadow Republican President Donald Trump’s claim of victory following the conclusion of Special Counsel Robert Mueller’s probe of Russian interference in the 2016 presidential election. The legal filing gave Democrats a natural opening to focus on an issue they say is more important to voters than the Mueller investigation.

“We always felt that the issues that affect average Americans - healthcare, climate change, jobs - (are) far more important to them, and to us, than what happens in an investigation,” Senate Minority Leader Chuck Schumer told reporters.

Democrats made defending Obamacare a powerful messaging tool in the run-up to the November’s elections, when opinion polls showed eight in 10 Americans wanted to defend its most popular benefits, including protections for insurance coverage for people with pre-existing conditions. The strategy paid off, and Democrats won a 38-seat majority in the House of Representatives.

The 2010 healthcare law, seen as the signature domestic achievement of Trump’s Democratic predecessor, Barack Obama, has been a flashpoint of American politics since its passage, with Republicans, including Trump, repeatedly attempting to overturn it.

FILE PHOTO - A sign on an insurance store advertises Obamacare in San Ysidro, San Diego, California, U.S., October 26, 2017. REUTERS/Mike Blake

Previously, the Trump administration had said portions of Obamacare should be struck down and others should survive, including a state-led expansion of Medicaid health insurance for the poor. Trump had said he would not cut that aspect when campaigning for the White House, although his 2020 budget proposal slashed the program’s funding.

“The Republican Party will soon be known as the party of healthcare - you watch,” Trump told reporters upon arriving at U.S. Capitol on Thursday for lunch with Republican lawmakers. He offered no details.

One of the first moves Democrats made after regaining control of the House was intervening in the Texas lawsuit to rebut the claim brought by a coalition of 20 Republican-led states. Texas, Alabama, Florida and the 17 other states had said a Trump-backed change to the U.S. tax code made Obamacare unconstitutional.

“Americans raised their hand and said, ‘We want the Affordable Care Act protected, therefore we’re going to vote for the party that says it will protect and preserve and, yes, even expand the Affordable Care Act,’” Representative Steny Hoyer, the No. 2 House Democrat, said at a Tuesday news conference.

Obamacare survived a 2012 legal challenge at the Supreme Court when a majority of justices ruled the individual mandate aspect of the program, which requires individuals to buy insurance or pay a penalty, was a tax that Congress had the authority to impose.

In December, U.S. District Judge Reed O’Connor ruled that after Trump signed a $1.5 trillion tax cut package passed by Congress last year that eliminated the penalties, the individual mandate could no longer be considered constitutional.

“The taxing authority by Congress that led to the legitimate reason for Obamacare being upheld is gone,” White House aide Kellyanne Conway told reporters on Tuesday. “We now have a mandate without a penalty.”

Conway said Obamacare had not been a “magic elixir” since more than 27.4 million non-elderly individuals in the United States were without health insurance in 2017, according to the Henry J. Kaiser Family Foundation.

A group of 17 mostly Democratic-led states including California and New York on Monday argued in court papers that the law was constitutional because the individual mandate is a “lawful choice between buying insurance or doing nothing.”

About 11.8 million consumers nationwide enrolled in 2018 Obamacare exchange plans, according to the U.S. government’s Centers for Medicare and Medicaid Services.