Don Humbertson, a 64 year old lung cancer surviver, is examined by Dr. Wade Harvey at the Clay-Battelle Community Health Center March 21, 2017 in Blacksville, West Virginia.

The Trump administration declared that it will no longer defend the Affordable Care Act from a challenge filed by 20 states in a brief filed Thursday night because it agrees that the law's individual mandate is unconstitutional and that key parts of the act — including the provisions protecting those with pre-existing conditions — are invalid.

President Trump has long declared the ACA, also known as Obamacare, to be a "disaster" and the brief is the latest attempt by his administration to weaken President Barack Obama's signature health-care law.

But the move also upends a longstanding legal and democratic norm that the executive branch will uphold existing laws.

"I am at a loss for words to explain how big of a deal this is," said University of Michigan law professor Nicholas Bagley in a blog post.

Bagley, a former Justice Department attorney, said the DOJ has a "durable, longstanding, bipartisan commitment" to defending the laws passed by Congress as long as there is a legitimate "non-frivolous" argument to be made in its defense.

"The Justice Department has an obligation to defend the law and it has refused to do so because it dislikes this particular law," Bagley told USA TODAY.

He said the only explanation for the brief is that the administration decided its "dislike for the Affordable Care Act outweighed its respect for the rule of law."

Bagley said the brief reveals the "depth of institutional decay at the Department of Justice" and he expressed profound concern about the precedent it sets.

"It suggests that future administrations can pick and choose which laws they're going to enforce," Bagley said.

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In a three-page letter to House Minority Leader Nancy Pelosi, D-Calif., Attorney General Jeff Sessions acknowledged that "the Executive Branch has a longstanding tradition of defending the constitutionality of duly enacted statutes if reasonable arguments can be made in their defense." But Sessions said this was a "rare case" and not unprecedented.

Bagley concedes that the Obama administration decided not to defend the Defense of Marriage Act, but said that this case "could not be more different" because it centers on a technical issue rather than "fundamental values."

The Trump administration argues that because the new tax law eliminates the penalty for not buying insurance, the Supreme Court's previous ruling permitting the mandate as a tax no longer applies.

The Justice Department does not join the 20 states in the lawsuit in saying that this invalidates the entire law. Rather, in the brief, the Justice Department says that only the section of the law dealing with the individual mandate is unconstitutional.

However, the Trump administration believes that the provision of the ACA guaranteeing affordable rates to those with pre-existing conditions must be thrown out with the individual mandate.

"Otherwise individuals could wait until they become sick to purchase insurance, thus driving up premiums for everyone else," Sessions said in his letter to Pelosi.

"Tonight, the Trump Administration took its cynical sabotage campaign of Americans' health care to a stunning new low," House Minority Leader Nancy Pelosi, D-Calif., said in a statement. "Once again, Republicans are trying to destroy protections for Americans with pre-existing conditions. The Trump Administration is perpetuating the same cruel vision of higher costs and less coverage that House Republicans voted for in the monstrosity of Trumpcare."

"The Trump administration once again is allowing premiums to go up and middle class Americans to pay more," Senate Minority Leader Chuck Schumer said in a tweet. "The ACA is the law of the land and DOJ should defend it."

The Washington Post reported that three career Justice attorneys involved in the case — Joel McElvain, Eric Beckenhauer and Rebecca Kopplin — withdrew before the filing at 6 p.m. Thursday, an unusual time for such an action.

Bagley said, "That's almost unheard of. These are lawyers who have made arguments they personally disagreed with countless times. They're civil servants. They're good soldiers. Yet they could not sign it. That's how far out the administration's position is."

Schumer Tweet

If the court agrees with the Justice Department's argument to toss out part of the law that protects individuals with existing medical condition, that could affect millions of Americans who buy insurance directly from the marketplace.

More than one in four adults under the age of 65 have existing medical conditions, according to Kaiser Family Foundation.

Before the Affordable Care Act marketplace launched in 2014, people with chronic diseases who tried to buy their own insurance were routinely denied coverage based on their medical history.

Bagley said that the Trump administration's move does not signal the end of Obamacare, although it does further cloud its future. States like California will take up the law's defense and the legal battle will continue.

While the case still must play out before the nation's high court, the Justice Department's decision to not defend the law could create some uncertainty for health insurers that sell plans on the Affordable Care Act marketplace, according to Larry Levitt, a Kaiser Family Foundation senior vice president who closely monitors the health law.

"When insurance companies face uncertainty, they increase premiums," Levitt tweeted Thursday evening.

After the latest attempt to repeal the health law failed with Arizona Sen. John McCain's dramatic "thumbs down" vote last July, the Trump administration has taken other steps that Obamacare advocates say weaken the law.

The administration last fall ended a key payment to insurers that helped lower the cost of insurance for low-income Americans who purchased marketplace insurance.

More recently, the administration also sought to loosen rules on short-term health insurance plans that are less expensive but don't carry the same consumer protections as Obamacare plans. Experts believe that could entice healthier people to exit the marketplace in favor of less-expensive, short-term plans, leaving the marketplace with a sicker population that could drive insurance costs higher.