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 sent 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.) on Thursday defending the Department of Justice's (DOJ) rationale for not defending the Affordable Care Act, also known as ObamaCare.

"As you know, the Executive Branch has a longstanding tradition of defending the constitutionality of duly enacted statutes if reasonable arguments can be made in their defense," Sessions wrote.

"But not every professionally responsible argument is necessarily reasonable in this context," he continued, adding this is "a rare case where the proper course is to forgo defense" of the law.

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The department argued in court on Thursday that key components of the Obama-era law are unconstitutional, siding in large part with a challenge to the law from 20 GOP-led states.

The DOJ points to the Supreme Court's ruling in 2012 that upheld ObamaCare's individual mandate — that people have insurance or face a tax penalty — as constitutional under Congress's taxing power.

After Congress repealed the mandate penalty as part of last year's tax bill, the GOP-led states and the DOJ argue the mandate itself is no longer a tax and is now invalid.

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

Sessions's move marks a break for the DOJ, which typically defends federal laws when they are challenged in court.

The move shows the Trump administration is not willing to defend the law, which it strongly disagrees with.

However, this is not the first time an administration has broken with precedent.

Former President Obama's Justice Department declined to defend the Defense of Marriage Act, which defined marriage as being between a man and a woman, in 2011.

Peter Sullivan contributed.