With the retirement of Justice Anthony M. Kennedy and the likelihood that President Trump will choose a more conservative replacement, Chief Justice John G. Roberts Jr. is poised to play a pivotal role on the court he has headed for 13 years.

Roberts has been content to play the long game, moving the court to the right with incremental steps. But now, with more conservative colleagues on one side and liberals on the other, Roberts will have the ability to supply the deciding fifth vote and dictate the terms of the deal.

“With four presumably reliable conservative votes, he’ll be in a unique position to decide where, how far, and how fast, the court goes,” said Gregory G. Garre, who was a solicitor general under President George W. Bush.

“This will put the chief in more of a swing justice role, and make him one of the most powerful chief justices in recent history.”

Roberts has joked in the past that it is a historical accident that the chief justice gets only one vote, just like the eight other members. So like the others, he, too, was often in need of Kennedy’s vote.

Kennedy’s exit “puts him in an extraordinarily privileged position of power,” said Daniel Epps, a Washington University law professor who monitors the Supreme Court for his podcast, First Mondays.

“If it wasn’t the Roberts court already, it is the Roberts court now, and it will be for a generation.”

With Roberts as the median justice, one of the most conservative Supreme Courts in history will almost surely move further to the right. And if the chief justice’s past is prologue, that could mean more restrictions on abortion rights, affirmative action contained or ended, gay rights more closely scrutinized, and states freer to alter voting laws and redistricting without judicial oversight.

It is a long-sought goal for conservatives, for whom Kennedy often was an obstacle.

Liberals have taken an apocalyptic view of a Supreme Court without Kennedy. By the time the court reconvenes in October, five justices will be conservatives nominated by Republican presidents. Two of the court’s four liberals will be octogenarians, and reinforcements are not on the way.

Kennedy usually sided with conservatives. But recent victories for the liberals on same-sex marriage, abortion accessibility and affirmative action were possible only because he joined them.

Roberts was on the other side, often with an adamant, stinging rebuke.

When Kennedy wrote the 2015 decision that found a constitutional right for same-sex couples to marry, Roberts said in his dissent that gay couples and their supporters should “by all means celebrate today’s decision.”

But he added: “Do not celebrate the Constitution. It had nothing to do with it.”

“The lesson of 2018 seems to be it can always get worse,” said Elizabeth B. Wydra, president of the liberal Constitutional Accountability Center. “And it did.”

A court in which Kennedy is replaced by another Trump choice “is also likely to encourage conservative legal activists to shoot for the stars — look for cases seeking to overrule Roe v. Wade, reverse or undermine LGBTQ rights, including marriage equality, and erode the progress toward racial justice that the civil rights movement has fought tirelessly to have recognized by an often recalcitrant court,” she said.

Such bold action is not usually Roberts’s style, and the Supreme Court is institutionally averse to overturning precedents, a legal principle called stare decisis. Roberts’s preferred path, his defenders and detractors say, involves limiting the court’s precedents rather than reversing them.

So he fulfilled his opposition to parts of the Voting Rights Act by striking a section at its heart, rather than the entire law. He infuriated the right — Trump during the campaign called him “an absolute disaster” — by finding a narrow way to uphold the Obama-era Affordable Care Act, while also eliminating a crucial element of it.

“Given that the chief also seems uniquely sensitive to public perception of the court, and concerns about the court overstepping its role, this will be a fascinating dynamic to follow for years to come,” Garre said.

In fact, he said, the court this term gave the public a taste of what a post-Kennedy court might look like.

Kennedy for once did not join liberals to form a 5-to-4 majority. His consistency led to decisions determined by the court’s conservatives, including:

• upholding President Trump’s ban on travelers from certain Muslim-majority countries
• overturning a 41-year-old precedent to prohibit public-sector employee unions from collecting fees from nonmembers
• approving a Texas redistricting plan that a lower court had found discriminated against blacks and Hispanics
• finding that the law does not provide a bond hearing for those detained and facing deportation, even after months or years in detention
• ruling that pregnancy centers cannot be required to tell patients about the availability of state-offered services, including abortion.

Kevin Russell, a lawyer who frequently practices at the Supreme Court, said some of the court’s decisions this term called into question the court’s desired reputation for evenhandedness.

“One of the themes of the term was that what’s good for the goose is not necessarily good for the gander,” he said.

His examples: “Abortion providers can be required to tell patients about adoption options, but crisis pregnancy centers cannot be required to tell their clients about abortion options. The First Amendment provides no protection when public sector workers complain about their pay individually, but protects an individual worker from having to share in the cost of a union trying to bargain about wages collectively.”

And the court ruled for a Colorado baker who did not want to create a wedding cake for a same-sex couple because it said a commission member made remarks hostile to the baker’s Christianity. But it “basically ignores President Trump’s tweets in upholding the travel ban,” Russell said.

“The court gives reasons for all these asymmetries, which may or may not be persuasive to people,” he said. “But the unfortunate fact is that in each instance, the differential treatment occurred in politically charged cases and uniformly favored conservative causes. That will not help the court’s credibility as a nonpartisan institution.”

Leah Litman, a liberal law professor at the University of California at Irvine, said the justices need not overturn the same-sex marriage decision to undermine it. “They could recognize a right for religious objectors not to marry LGB individuals; not to serve them; not to provide them health care; not to allow them to adopt,” she wrote.

[Anthony Kennedy and the four Supreme Court rulings that changed gay life in America]

A decision about whether a business owner may refuse wedding services to a same-sex couple because it would violate his religious beliefs is one the justices left open this term for another time.

Michael McConnell, a former Republican-nominated federal judge and head of the Stanford Constitutional Law Center, said predictions about the new court are difficult.

He noted that this term especially revealed a split among the court’s liberals — Justices Ruth Bader Ginsburg and Sonia Sotomayor further to the left, Stephen G. Breyer and Elena Kagan closer to the middle.

“If Justice Kennedy is replaced with an interesting, relatively non-doctrinaire conservative (like the best names on Trump’s list) this could augur a more fluid court with a more substantive middle and fewer 5-4 splits,” he wrote in an email.

He also noted that the justices, as a group and as individuals, value independence.

“Every action seems to have a countervailing reaction,” McConnell said. “It would not surprise me to see a few of the conservative justices breaking more frequently with the liberal side.

“The court as an institution does not like to see itself as the instrument of an ideological movement.”