The Department of Justice (DOJ) argued in court Thursday that key parts of ObamaCare are now unconstitutional, siding in large part with a conservative challenge to the law.

The move is a break from the normal practice of the DOJ to defend federal laws when they are challenged in court. Under President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE, the department has opted not to defend a law that it strongly opposes.

Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE acknowledged in a letter to Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (R-Wis.) that the DOJ has a "longstanding tradition" of defending federal laws, but argued that this is "a rare case where the proper course is to forgo defense" of the law.

The lawsuit in question was filed in February by Texas and 19 other GOP-led states, arguing that ObamaCare is unconstitutional and should be overturned.

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Legal experts are deeply skeptical the challenge can succeed, and 17 Democratic-led states have already intervened to defend the law in the absence of DOJ action.

The DOJ argues that ObamaCare’s protections against people with pre-existing conditions being denied coverage or charged more should be invalidated, maintaining that the individual mandate that people have insurance or face a tax penalty is now unconstitutional.

The conservative states and DOJ point to the Supreme Court's 2012 ruling that upheld ObamaCare's individual mandate under Congress's taxing power. Now that Congress has repealed the mandate penalty as part of last year's tax bill – while technically keeping the mandate itself in place – they argue the mandate is no longer a tax and is now invalid.

They also argue that the key pre-existing condition protections cannot be separated from the mandate and should be invalidated. The DOJ argues the remainder of the law can stay.

The chances for that argument succeeding are viewed with deep skepticism by legal experts, in part because Congress itself indicated that the rest of ObamaCare could still stand without the mandate when it moved to repeal the tax penalty last year.

The case is currently before a federal district court judge in Texas, Reed O'Connor, who was appointed by former President George W. Bush.

Some supporters of ObamaCare view the DOJ's move more as a damaging break from precedent rather than an actual serious legal threat to ObamaCare, since the lawsuit is unlikely to succeed.