An extraordinary four years of disruption, division and partisan warfare finds Republicans on the threshold of a dream decades in the making: a Supreme Court with a seemingly unshakable conservative foundation.

If Judge Amy Coney Barrett is placed on the court by the GOP-controlled Senate, President Trump will have named a third of the Supreme Court and solidified a long-elusive 6 to 3 conservative majority.

One would have to go back to Richard Nixon to find a first-term president whose nominees have so reshaped its direction, and for decades to come.

That change is likely to mean a lower bar for laws that restrict abortion. It will bring higher scrutiny of gun regulation. The Affordable Care Act is on the docket again, and one member of the scant majority that upheld it in 2012 is gone. It will be a surprise if affirmative action and other race-conscious programs survive another challenge.

And whether the public continues to see the court as the government’s most functional branch — or if radical restructuring is pursued — is on the line as well.

Fate, and political hardball by Senate Majority Leader Mitch McConnell (R-Ky.), has presented Trump the chance to replace a legendary justice from the right, Antonin Scalia; the iconic leader of the left, Ruth Bader Ginsburg; and the man who for more than a decade occupied the pivotal spot in the middle, Anthony M. Kennedy.

The change has followed a calamitous path. Democrats claim Trump’s first appointee, Justice Neil M. Gorsuch, 53, occupies a “stolen” seat — McConnell refused to hold a hearing for then-President Barack Obama’s nomination of Judge Merrick Garland to fill the 2016 Scalia vacancy.

The confirmation of Justice Brett M. Kavanaugh, 55, was a wrenching, hard-to-watch drama that divided the nation over whether he assaulted Christine Blasey Ford as a teenager or was the victim of a left-wing smear aimed to keep him off the court.

And consider the current scenario: Barrett, the 48-year-old ideological opposite of the woman she would replace, could be locked in to her lifetime appointment days before — or even after — her political benefactor, and his quest for a second term, is rejected by voters.

The justices testify endlessly that they put aside partisan impulses when they put on the robe. So the president did the court no favor when he said his political future might depend on getting his nominee confirmed.

“I think this will end up in the Supreme Court. And I think it’s very important that we have nine justices,” Trump said during the buildup to the nomination. “It’s better if you go before the election, because I think this scam that the Democrats are pulling — it’s a scam — the scam will be before the United States Supreme Court. And I think having a 4-to-4 situation is not a good situation.”

It is difficult for the public to see the court as politically neutral “when a primary reason being given in favor of an expedited Senate confirmation hearing is . . . so the new justice can be there in time to vote in a way that will ensure the reelection of the president who just nominated them,” said Richard J. Lazarus, a Harvard University law professor who studies the court.

Trump’s calculations seem to put Chief Justice John G. Roberts Jr., nominated by President George W. Bush in 2005, in the side against him.

Roberts in the most recent term joined with the court’s liberals to strike a restrictive Louisiana abortion law; declare the administration had not followed the law in trying to abolish the program protecting undocumented immigrants brought to the United States as children; and, along with Gorsuch, reject the administration’s view that federal anti-discrimination law does not protect LGBTQ workers.

But likely the most stinging for Trump was that Roberts wrote the opinions dismissing his claims that he had immunity while in office from complying with investigations by congressional committees and a New York prosecutor, each seeking the president’s personal financial records.

“No citizen, not even the president, is categorically above the common duty to produce evidence when called upon in a criminal proceeding,” Roberts wrote in the prosecutor case, and he stitched together seven justices to agree with that outcome, including Gorsuch and Kavanaugh.

But Roberts is hardly a liberal.

“The chief justice seemed to be guided last term by institutional concerns in particular in shifting to the left in some of the more contentious cases, but he has a conservative track record by almost any measure,” said Gregory G. Garre, who represented the government at the Supreme Court as solicitor general under Bush.

The common wisdom, attributed to the late justice Byron White, is that each new justice creates a new court. But each variation is known as the Roberts court.

The intriguing question always has been what Roberts’s role would have been if he were not the chief justice, but simply an associate justice.

“Have we seen the real John Roberts?” asked Melissa Murray, a liberal New York University law professor who helps run a podcast about the court. Like others, she finds it hard to imagine him aligning with liberals if it means he is on the losing side of a 5-to-4 vote.

The chief justice’s vote counts no more than any other justice’s, and his colleagues have made it clear they work with him, not for him. But the chief justice does have one institutional power: He can either write the opinion in a case when he is in the majority, or decide who does.

He has used it at times to pump the brakes when more conservative justices would like to advance the law further and faster, or to keep liberals in line when their only chance to prevail is with his vote.

The chief justice’s approach, which last term avoided deciding many of the court’s most high-profile cases based on 5-to-4 ideological votes, proved popular with the public. A Gallup poll showed a majority of Democrats and Republicans approved of the court’s performance, a rare feat in this polarized political climate.

Roberts’s influence will be diminished if he is not the person in the middle between four justices more conservative than he is, and four more liberal.

Not all of his conservative colleagues are as concerned as he is about shifting the court’s jurisprudence swiftly; that is what worries liberals most about a new court erasing long-standing protections for minority groups or abortion rights.

The court’s longest-serving member, Justice Clarence Thomas, has been clear in his belief that the court should not be shy about overturning precedent when a majority thinks the original case was wrongly decided.

When it comes to constitutional cases, the woman who could become Thomas’s junior partner seems to be on the same wavelength.

“I tend to agree with those who say a justice’s duty is to the Constitution,” Barrett wrote in a 2013 article for the Texas Law Review, “and that it is thus more legitimate for her to enforce her best understanding of the Constitution rather than a precedent she thinks clearly in conflict with it.”

There is little reason to believe Roberts would be unhappy with additional conservative outcomes, but he prefers an incremental pace that prepares the public for change.

“A 6-to-3 court would shift power from the chief justice as the deciding vote in the more contentious cases to the more conservative justices. And Justice Kavanaugh may find himself as the crucial fifth vote in cases,” Garre said.

“But Justice Kavanaugh, who aligned with the chief justice in more than 90 percent of the cases last term, seems to share the chief’s institutional concerns.”

Predictions about the court are difficult until the results of the election are known. “The idea that the court operates in a vacuum is just fiction,” Murray said.

And the election of Democrat Joe Biden and a Democratic Senate “would really accelerate and amplify the calls for reform,” she said, such as adding seats to the court to counter conservative gains.

Biden has said he is against that — Ginsburg was, too — but he would face enormous pressure. Liberal activists say the court-packing already has been done, with Trump filling the seat that became open during Obama’s term and the president’s rush to nominate a replacement for Ginsburg even though voting in the presidential election has begun.

The unstable atmosphere might be reason for caution, at least initially.

“While the chief justice has seemed particularly sensitive to institutional concerns, it is likely that the court as a whole would be sensitive to concerns about an abrupt shift in jurisprudence as it transitions from the current vacancy,” Garre said.

“In that regard, the court is likely to look for ways to decide cases narrowly or incrementally, especially during this transition period.”