When Chief Justice John Roberts delivered the decisive vote in 2012 that upheld Barack Obama’s signature achievement in office, the Affordable Care Act, he reportedly did so following after a month-long campaign by fellow conservatives to try to get him to join their side.

His decision to side with liberal colleagues inspired ire on the right but it also cemented the chief justice’s role as the leader of his own court.

That was then.

Last week, as the supreme court began a new session that will include rulings on abortion, gun rights, and torture, Roberts no longer holds the coveted role of sitting in the court’s ideological center.

The death of Ruth Bader Ginsburg and confirmation of Amy Coney Barrett means Roberts – who has sought to portray himself as an “institutionalist” seeking to protect the court’s legitimacy – no longer has the sole power to cast the deciding vote in any ruling.

Who, then, does control the Roberts court?

Legal experts disagree over the voice that will ultimately be seen as driving the majority’s opinions this session, which has been described as “the most important” one the court has faced in “decades”, and likely to be “tumultuous”.

The most-watched case will involve a Mississippi law that outlaws abortion after 15 weeks of pregnancy, in what is seen as a direct challenge to Roe v Wade, the landmark decision that legalized abortion.

In order for liberals to win any major ruling, they would require not just Roberts, a pro-business conservative, to side with them, but for another conservative justice to join him for a 5-4 win over the other conservatives.

“Roberts is only marginally in charge to the extent that he can bring Kavanaugh or Barrett with him,” said Josh Blackman at the South Texas College of Law Houston.

For Blackman, it’s now Clarence Thomas, the most conservative member of the court, who rules. “I think we are living in Justice Thomas’s world. He is always thinking ahead several steps and has built an army of supporters,” he said.

Another court watcher, Elie Mystal at the Nation, said that it was true that conservative activists have – for decades – sought to put Thomas at the centre of conservative judicial movement because he has been a reliable supporter of the Republican agenda. But his extreme views have also made him an outlier.

“Maybe this term we will get the first impactful majority opinion by Clarence Thomas, because we have not seen it yet [in nearly 30 years],” Mystal said.

The more likely outcome, Mystal argued, is that Neil Gorsuch would emerge as the intellectual conservative heavyweight, which he said would more often than not “spell doom” for liberals.

“Roberts’s ambition is to keep the law as narrow as possible and keep as much of the court’s legitimacy as possible while twirling ever toward the Republicans’ agenda. Kavanaugh likes beer. Amy Coney Barrett likes Jesus. These are fundamentally narrow positions,” he said, with a chuckle.

“Gorsuch wants to fundamentally change the law and reframe the way we think about the law. Gorsuch wants to do horrible things, but ambitious things. He’s got the intellectual tools to accomplish it.”

Most legal experts say that even if conservatives do not fully reverse Roe v Wade, that the abortion decision could be gutted in a way that in effect will allow states to make abortion so severely restricted as to be illegal.

Garrett Epps, a professor at the University of Baltimore School of Law, noted that at least two justices – Barrett and Samuel Alito – seemed “unusually defensive” in recent remarks in which both defended the court against allegations of partisanship.

“I think the thing to watch is whether Roberts can pick off one of the members of the [conservative] super majority to have the result come out differently than the hard right would want,” said Epps. “I think it may not be his court, but his role is by far the most interesting of any of the justices because he is – and he alone – an institutionalist, in the sense that he really cares about the Supreme Court and his place in history, more than he really cares about his own policy goals.”

While Roberts is “very, very conservative”, he is not – Epps added – a “movement conservative”. Nor is he motivated by what Epps characterized as the “rage” of some of the other justices.

Franita Tolson, vice dean for faculty at the University of Southern California Gould School of Law, said it is doubtful that Roberts would ultimately be able to convince any other justices to try to uphold Roe v Wade, or whether he wants to.

“The cases in this term do not give him an easy out to be an institutionalist and trying to build a coalition, versus him indulging his personal preferences. And overturning Roe v Wade might be his personal preference. And if it is, it’s still the Roberts court.”