WASHINGTON − Gun control advocates had been cautiously optimistic the Supreme Court would uphold Biden administration rules to regulate otherwise untraceable “ghost guns.”

But despite the justices sounding in last month’s oral arguments as if they might side with the White House, the incoming Trump administration could rescind the requirement that guns made from kits must have serial numbers and owners must pass background checks.

“Obviously, we’re very worried because of the major public safety and public health concerns,” Douglas Letter, chief legal officer at the Brady Center to Prevent Gun Violence, said of how President-elect Donald Trump’s victory could affect the case.

Whiplash may be even more likely on another big question before the high court: whether states can ban puberty blockers and hormone therapy for transgender minors.

In early December, the Justice Department will try to convince the court that such bans are unconstitutional.

But it’s likely to be months before the justices issue their opinion. And before they do, a Trump Justice Department could tell the court its position has changed.

Transgender care: A top Trump issue

Trump, who has said he wants to “stop the chemical, physical and emotional mutilation of our youth,” made opposition to transgender rights a central theme of his campaign.

“I think we’ve seen so far from the Trump campaign a very strong commitment to protecting kids from these harmful drugs and surgeries,” said Matt Sharp, senior counsel at the Alliance Defending Freedom which filed a brief defending a Tennessee ban on transgender treatments for minors.  “We’ll see ultimately what the administration does with it, if they take a formal position or if they allow the case to move forward.”

The Trump campaign did not respond to requests for comment about positions the incoming administration may take on questions before the court.

In addition to the cases already being argued, others headed to the justices in which the change of administrations is expected to make a big difference involve abortion, preventive care coverage under the Affordable Care Act, environmental protections and other regulatory actions.

“I have no doubt the administration will end up with major changes in these policies and their positions before the Supreme Court,” said Adam Winkler, a professor in the UCLA School of Law.

Whiplash in the courts, from Trump to Biden to Trump

During the first Trump administration, the Justice Department early on reversed its previous positions when arguing four high-profile cases involving the purging of voter rolls, labor protections, union fees and the enforcement authority of Securities and Exchange Commission judges.

The reversals were “abrupt and appeared strikingly at odds with institutional norms,” Michael Dreeben, a former deputy solicitor general wrote in a 2021 analysis for the Yale Law Journal. That prompted speculation that the justices would look askance at the side-switching.

In fact, Dreeben noted, the court sided with Trump’s Justice Department in all four cases.

When President Joe Biden took office, the Justice Department reversed position on a challenge to the Affordable Care Act. The Trump administration had backed Texas’ effort to dismantle the law. The Supreme Court ultimately found that Texas couldn’t show it had been sufficiently harmed by the law and dismissed the challenge without deciding the underlying constitutional issue.

The Biden administration also ended Trump immigration policies that were being litigated all the way up to the Supreme Court.

Now, the incoming Trump administration could do the same for an abortion issue that could be headed back to the justices.

The Biden administration has been challenging Idaho’s strict abortion ban as being in conflict with a federal law that requires emergency rooms to provide stabilizing care. That includes abortions, the administration argues, if needed to prevent serious health consequences.

The Supreme Court in June sent the case back to an appeals court for further review. But even if the San Francisco-based 9^th U.S. Circuit Court of Appeals sides with the Biden’s Justice Department, the Trump administration could drop the challenge if Idaho appeals that decision to the Supreme Court.

It’s less clear what would happen if the incoming administration no longer wants to challenge states’ bans on gender-affirming care for minors.

If the Justice Department notifies the court that its position has changed, the justices could ask the department to lay out its new legal argument or could let the American Civil Liberties Union – which is representing the families challenging Tennessee’s ban – to solely represent that side.