The Supreme Court’s conservative majority on Wednesday appeared poised to side with Mississippi in its bid to uphold a 15-week abortion ban, a ruling that would erode decades-old precedent protecting the right to an abortion before viability.

The justices heard oral arguments in the case, Dobbs v. Jackson Women’s Health Organization, that directly challenged the abortion rights precedent established by the 1973 Supreme Court ruling Roe v. Wade and reaffirmed by the Planned Parenthood v. Casey decision in 1992.

The case centers on a Mississippi law that would ban almost all abortions after 15 weeks of pregnancy. Lower courts blocked the law, ruling that it violates Roe and Casey, which protect abortion before the point of fetal viability — around 24 weeks of gestation — and require that laws regulating abortion not pose an “undue burden.”

The Mississippi case marks the most significant challenge to abortion rights in decades.

The 6-3 conservative majority questioned Julie Rikelman, an attorney with the Center for Reproductive Rights arguing in favor of Roe and Casey, about the viability threshold.

“Viability, it seems to me, doesn’t have anything to do with choice,” Chief Justice John Roberts said. “If it really is an issue about choice, why is 15 weeks not enough time?”

Justice Brett Kavanaugh, one of former President Donald Trump’s three appointees to the court, expressed skepticism that the interests of pregnant women and fetuses can both be accommodated.

“The problem, I think the other side would say, and the reason this issue is hard, is that you can’t accommodate both interests. You have to pick. That’s the fundamental problem,” Kavanaugh told U.S. Solicitor General Elizabeth Prelogar, who also argued against Mississippi.

“And one interest has to prevail over the other at any given point in time, and that’s why this is so challenging, I think. And the question then becomes, what does the Constitution say about that?” Kavanaugh said.

The three liberal justices on the nine-member bench, meanwhile, sounded alarms that reversing Roe and Casey would destroy the public perception of the high court, and thus the institution itself.