WASHINGTON — Much of the discussion of a Mississippi law that would ban abortions after 15 weeks of pregnancy has suggested that the Supreme Court faces a binary choice: it could strike down the law and fully reaffirm Roe v. Wade as the law’s challengers want, or it could gut the idea that the Constitution protects abortion rights at all, as Mississippi has urged.

But during the Supreme Court’s oral arguments on Wednesday in a lawsuit challenging the law, Chief Justice John G. Roberts Jr. appeared to be exploring whether he could find something of a middle ground — one that would allow the court to uphold the Mississippi law without also proclaiming that the Constitution offered no protection of any right to an abortion.

Under Roe and a 1992 case that reaffirmed it, Planned Parenthood v. Casey, it is unconstitutional to ban abortions before “viability,” the point at which a fetus can survive outside the womb, which is usually about 24 weeks into pregnancy. On Wednesday, Chief Justice Roberts repeatedly returned to whether the cutoff could be earlier.

“If you think that the issue is one of choice, that women should have a choice to terminate their pregnancy, that supposes that there is a point at which they’ve had the fair choice,” he said, and went on to question why 15 weeks would be an inappropriate line. “Because viability, it seems to me, doesn’t have anything to do with choice. But, if it really is an issue about choice, why is 15 weeks not enough time?”

How Chief Justice Roberts handles the case could have outsized importance. Compared to some of the other five members of the court’s conservative bloc, he is broadly seen as more likely to be concerned about the institutional impact on the court if it makes a wrenching and politically contentious change in the law. He also has the power to assign himself to write the opinion if he votes with the majority.

Known for crafting narrow and incremental decisions, Chief Justice Roberts distinguished on Wednesday between an outright ban on abortion and a ban on the procedure that was stricter than the current standard. At one point, he remarked that he thought moving the cutoff line to 15 weeks — nine weeks earlier than where it is now — was “not a dramatic departure from viability.”

He also told the solicitor general, Elizabeth Prelogar, who was supporting the challengers to the law, that the arguments she had made regarding problems that would result from overruling Roe “sounded to me like being based on a total prohibition.” He said he wondered how strong those arguments “would be if there isn’t a total prohibition.”

Similarly, addressing arguments about the impact of abortion rights on women and their ability to have an equal place in society, Chief Justice Roberts asked Julie Rikelman of the Center for Reproductive Rights, a lawyer for the challengers, whether it would make a huge difference to move up the deadline to make a decision from 24 to 15 weeks.

“Are you suggesting that the difference between 15 weeks and viability are going to have the same sort of impacts?” he asked.

But both Ms. Prelogar and Ms. Rikelman urged the court to keep the line at fetal viability, which Ms. Rikelman said was “objectively verifiable and doesn’t require the court to resolve the philosophical issues at stake.”

Chief Justice Roberts also questioned Mississippi’s solicitor general, Scott Stewart, about whether the “viability” line was really central to Roe, as the court said it was in the 1992 case reaffirming abortion rights. The chief justice noted that fetal viability had not been part of the legal arguments in Roe, and noted that the once-confidential papers of Roe’s author, Justice Harry A. Blackmun, suggested that putting the line at viability was arbitrary.

A major issue is whether it is proper for the Supreme Court to reopen the abortion rights question under the legal doctrine of stare decisis — under which it should hesitate to overturn precedents — even if its current members would have decided those cases in a different way. The chief justice’s line of reasoning might allow him to claim that eliminating the viability line did not really amount to overturning Roe.

But Chief Justice Roberts did not offer a fleshed-out rationale for why allowing states to ban abortion at an earlier point in pregnancy — but apparently still not allowing them to prohibit the procedure entirely — would be more principled than keeping it at the point of fetal viability. And the idea with which he seemed to be playing received pushback.

At one point, he put forward the proposition that most countries that permit abortion have an earlier cutoff line, and said the United States was like North Korea and China in using fetal viability as the standard. But Ms. Prelogar, the solicitor general, told him his understanding of other countries’ laws was incorrect.

In fact, she said, while it is true that many Western countries that are similar to the United States have nominally earlier limits like 12 or 18 weeks, in reality they also allow abortions up to viability because those limits contain sweeping exceptions for “broad social reasons, health reasons, socioeconomic reasons.”

The arguments also focused on whether upholding the Mississippi law without also declaring there is no constitutional right at all to an abortion would be a middle ground, or rather would just open the door to a floodgate of stricter abortion bans and lawsuits challenging them.

Ms. Rikelman warned Chief Justice Roberts that there would be no principled stopping point if the court upheld the Mississippi law, so states would “rush to ban abortion at virtually any point in pregnancy.” Ms. Prelogar similarly predicted that if the Mississippi law was upheld, states would immediately enact bans at 10, eight or six weeks to push further.

But Chief Justice Roberts brushed aside that concern and a similar objection by saying, “I’d like to focus on the 15-week ban,” and that “the thing that is at issue before us today is 15 weeks.”

At another point, he sought an explanation from Mr. Stewart about why Mississippi had shifted from focusing on whether the viability cutoff was constitutionally required — as it did in its petition asking the Supreme Court to take up the case — to arguing more sweepingly for Roe and Casey to be flatly overruled.

Mr. Stewart told him that the state’s legal team shifted gears in part because the more modest argument that viability was not the appropriate standard had “lost in every court of appeals.”