WASHINGTON — The Supreme Court has voted to strike down the landmark Roe v. Wade decision that has guaranteed the right to abortion for nearly a half-century, according to a leaked draft opinion from February published online Monday night by Politico.

In the draft opinion, written by Justice Samuel A. Alito Jr., a majority of the court voted to overturn Roe, according to Politico. Justice Alito called it wrongly decided and said the contentious issue, which has animated political debates in the United States for more than a generation, should be decided by politicians, not the courts.

“We hold that Roe and Casey must be overruled,” Justice Alito writes in the document, labeled the “Opinion of the Court,” referring to a second decision that reaffirmed Roe. “It is time to heed the Constitution and return the issue of abortion to the people’s elected representatives.”

The draft posted by Politico is consistent with the Supreme Court’s published opinions in ways large and small, including structure, length, typography and how legal citations are rendered. Its assertive and sometimes slashing tone reads very much like other major opinions from Justice Alito.

The release of the 98-page document is unprecedented in the court’s modern history: Early drafts of opinions have virtually never leaked before the final decision is announced, and never in such a consequential case. And early drafts of opinions often change by the time the decision from the court is announced.

Shortly after the article was published Monday night, Politico’s editor in chief, Matthew Kaminski, and its executive editor, Dafna Linzer, sent an email to newsroom employees emphasizing its authenticity. In the memo, Mr. Kaminski and Ms. Linzer said that the article underwent “an extensive review process,” describing it as “plainly news of great public interest.”

Asked for reaction to the apparent leak, a Supreme Court spokeswoman said the court had no comment.

If the justices announce a decision along the lines of the early, leaked draft, it would be a seismic change in American law and politics, coming just months before congressional midterm elections that will decide who controls power on Capitol Hill.

Abortion has long split the two parties — and the country — though it had receded as a central issue in presidential elections even while remaining a galvanizing issue for many. A court decision along the lines of the one in the early draft could incite new political battles in Congress and in states across the country about whether and how the procedure should be limited.

The Politico report said the justices voting to support Justice Alito’s opinion were Clarence Thomas, Neil M. Gorsuch, Brett M. Kavanaugh and Amy Coney Barrett. The news organization said Justices Stephen G. Breyer, Sonia Sotomayor and Elena Kagan were working on dissents. It was not clear how Chief Justice John G. Roberts Jr. planned to vote.

Roe v. Wade, which established a constitutional right to abortion in a landmark 1973 case, has been a centerpiece of American jurisprudence ever since. In the language of the court, it has been a precedent that cemented the basic rights of women to have access to a legal abortion. Over the years, the court has accepted restrictions on that right, but has not wavered from the basic legal standard set out by Roe.

The current court — which has six conservative justices and three liberal ones — has provided indications over the past year that it may be willing to reconsider that position.

During Supreme Court arguments in December, conservative justices indicated a willingness to scale back, if not undo, the federal abortion protections and leave most of the regulation up to individual states.

In more than two dozen conservative states, lawmakers have prepared bills that would effectively outlaw abortion if the court overturns Roe v. Wade. If the court embraces Justice Alito’s draft opinion as its final position, it would clear the way for those bills to quickly become law.

The draft opinion makes familiar arguments against Roe. It says that the Constitution is silent about abortion and that nothing in its text or structure supports a constitutional right to abortion. Roe, the draft continues, is so egregiously wrong that it does not deserve to be retained as a precedent. The proper approach, the draft says, is to return the question to the states.

The Mississippi law challenged in the case bans abortions after 15 weeks of pregnancy, and Justice Alito could have adopted a middle-ground approach advocated by Chief Justice Roberts when the case was argued in December: to sustain the law and leave questions about the fate of Roe for another day.

According to Justice Alito’s draft, a majority rejected that approach.

If the draft opinion or something like it is ultimately issued, it will produce rifts at the court that could test its legitimacy.

At the argument, the court’s three liberal members said that overruling Roe soon after a change in the court’s membership would damage the court’s authority. Indeed, Justice Sonia Sotomayor said, doing so would pose an existential threat.

“Will this institution survive the stench that this creates in the public perception that the Constitution and its reading are just political acts?” she asked.

“If people actually believe that it’s all political, how will we survive?” she asked. “How will the court survive?”

The leak of the draft opinion sent a jolt through Washington Monday night. The revelations from the draft opinion once again place the nine justices at the center of one of the most contentious issues in American life.

But the leak may also be the starting gun on a fierce, new political debate even before the justices issue a final ruling.

Conservatives who oppose abortion rights quickly hailed Justice Alito’s conclusions as the correct ones for the country, praising him for legal reasoning that they have been arguing for decades in the court of public opinion.

“We don’t know whether rumors of the end of Roe are accurate yet, but we know that ending Roe is the right decision, returning the issue to ‘we the people’ from a few judges with an agenda,” said Kristan Hawkins, the president of Students for Life of America. “You won’t find ‘abortion’ written in invisible ink in the Constitution, undiscovered until seven men saw it in 1973. Ending preborn human life is and has always been a judicial error.”

Senator Tom Cotton, Republican of Arkansas, tweeted Monday night that “Roe was egregiously wrong from the beginning & I pray the Court follows the Constitution & allows the states to once again protect unborn life.”

But he also assailed the leak of the draft opinion, saying the Supreme Court and the Justice Department “must get to the bottom of this leak immediately using every investigative tool necessary.”

Democratic lawmakers and liberal activists also criticized the leak. But many quickly seized on the news as a prime reason that voters should support Democrats in the fall elections.

“If this report is true, this Republican attack on abortion access, birth control and women’s health care has dramatically escalated the stakes of the 2022 election,” said Christie Roberts, the executive director of the Democratic Senatorial Campaign Committee. “At this critical moment, we must protect and expand Democrats’ Senate majority with the power to confirm or reject Supreme Court justices.”

Cecile Richards, who served as president of Planned Parenthood from 2006 to 2018, as Congress and state legislatures ramped up restrictions on reproductive health, said that “ending legal abortion will not end abortion. It simply will mean that women are no longer safe in this country, and that lies at the feet of the Republican Party.”

On Twitter Monday night, the news generated a debate about which political party might benefit from the early revelation of the court’s possible decision. Many argued that Democrats would use the report to energize their core voters.

Senator Patty Murray, Democrat of Washington and a member of her party’s leadership in the Senate, promised to do just that.

“After ringing these alarms for years now, it’s time to break the glass,” she wrote in a statement. “We need to fight back with everything we’ve got right now. The right to abortion is on the line, and I’ll never stop fighting to protect it.”

Reporting was contributed by Carl Hulse, Emily Cochrane, Elizabeth Dias from Washington, and Kate Zernike, Benjamin Mullin and Katie Robertson from New York.