NEW YORK — By today’s politically polarized standards, the Supreme Court’s momentous Roe v. Wade ruling was a landslide. By a 7-2 vote on Jan. 22, 1973, the justices established a nationwide right to abortion.

Forty years and roughly 55 million abortions later, however, the ruling’s legacy is the opposite of consensus. Abortion ranks as one of the most intractably divisive issues in America, and is likely to remain so as rival camps of true believers see little space for common ground.

Unfolding events in two states illustrate the depth of the divide. In New York, already a bastion of liberal abortion laws, Gov. Andrew Cuomo pledged in his Jan. 9 State of the State speech to entrench those rights even more firmly. In Mississippi, where many anti-abortion laws have been enacted in recent years, the lone remaining abortion clinic is on the verge of closure because nearby hospitals won’t grant obligatory admitting privileges to its doctors.

“Unlike a lot of other issues in the culture wars, this is the one in which both sides really regard themselves as civil rights activists, trying to expand the frontiers of human freedom,” said Jon Shields, a professor of government at Claremont McKenna College. “That’s a recipe for permanent conflict.”

On another hot-button social issue — same-sex marriage — there’s been a strong trend of increasing support in recent years, encompassing nearly all major demographic categories.

There’s been no such dramatic shift, in either direction, on abortion.

For example, a new Pew Research Center poll finds 63 percent of U.S. adults opposed to overturning Roe, compared to 60 percent in 1992. The latest Gallup poll on the topic shows 52 percent of Americans saying abortion should be legal under certain circumstances, 25 percent wanting it legal in all cases and 20 percent wanting it outlawed in all cases — roughly the same breakdown as in the 1970s.

“There’s a large share of Americans for whom this is not a black-and-white issue,” said Michael Dimock, the Pew center’s director. “The circumstances matter to them.”

Indeed, many conflicted respondents tell pollsters they support the right to legal abortion while considering it morally wrong. And a 2011 survey of 3,000 adults by the Public Religion Research Institute found many who classified themselves as both “pro-life” and “pro-choice.”

Shields, like many scholars of the abortion debate, doubts a victor will emerge anytime soon.

“There are reasonable arguments on both sides, making rationally defensible moral claims,” he said.

Nonetheless, the rival legions of activists and advocacy groups on the front lines of the conflict each claim momentum is on their side as they convene symposiums and organize rallies to commemorate the Roe anniversary.

Supporters of legal access to abortion were relieved by the victory of their ally, President Barack Obama, over anti-abortion Republican Mitt Romney in November.

A key reason for the relief related to the Supreme Court, whose nine justices are believed to divide 5-4 in favor of a broad right to abortion. Romney, if elected, might have been able to appoint conservative justices who could help overturn Roe v. Wade, but Obama’s victory makes that unlikely at least for the next four years.

Abortion-rights groups also were heartened by a backlash to certain anti-abortion initiatives and rhetoric that they viewed as extreme.

“Until politicians feel there’s a price to pay for voting against women, they will continue to do it,” said Cecile Richards, president of the Planned Parenthood Federation of America, a lightning rod for conservative attacks because it’s the leading abortion provider in the U.S.

In Missouri and Indiana, Republican candidates for the U.S. Senate lost races that their party initially expected to win after making widely criticized comments regarding abortion rights for impregnated rape victims. In Virginia, protests combined with mockery on late-night TV shows prompted GOP politicians to scale back a bill that would have required women seeking abortions to undergo a transvaginal ultrasound.

“All these things got Americans angry and got them to realize just how extreme the other side is,” said Jennifer Dalven, director of the American Civil Liberties Union’s Reproductive Freedom Project.

“This issue will remain very divisive,” she said. “But I do see this as a sea-change moment... The American public wants abortion to remain safe, legal and accessible.”

However, anti-abortion leaders insist they have reason for optimism, particularly at the state level.

In the past two years, following Republican election gains in 2010, GOP-dominated state legislatures have passed more than 130 bills intended to reduce access to abortion. The measures include mandatory counseling and ultrasound for women seeking abortions, bans on abortion after 20 weeks of pregnancy, curbs on how insurers cover the procedure, and new regulations for abortion clinics.

The ACLU and other abortion-rights groups are challenging several of the laws in court, notably the 20-week ban. Yet already this year, Republican leaders in Texas, Mississippi and elsewhere are talking about new legislative efforts to restrict abortion.

Mississippi’s Gov. Phil Bryant says he wants to end abortion in the state and is eager for the remaining clinic, the Jackson Women’s Health Organization, to close.

“My goal, of course, is to shut it down,” Bryant told reporters on Jan. 10. “If I had the power to do so legally, I’d do so tomorrow.”

The clinic is a steady target of anti-abortion protesters who take turns praying, singing hymns and confronting patients. Its administrator, Diane Derzis, says the three principal physicians on her staff have been unable to get admitting privileges at area hospitals due to pressure from the anti-abortion movement.

Such developments hearten Charmaine Yoest, president of Americans United for Life, one of the groups most active in proposing anti-abortion bills for state legislatures to consider.

“Within the context of Roe, we have been remarkably successful in terms of expanding the legal protection of human life,” Yoest said. “We’re working to make Roe irrelevant.”

Yoest’s optimism derives partly from her belief that young Americans are increasingly skeptical about abortion, though polls give mixed verdicts on this matter.

“It is really easy to explain the pro-life position to a child — it’s hard to explain to them why you should kill a baby before it’s born,” Yoest said.

Supporters of legal access to abortion dispute the notion of swelling anti-abortion sentiment among young people, but some activists do sense a gap in terms of political intensity.

“I have enormous hope in this millennial generation — they’re progressive, thoughtful and they identify in their pro-choice values,” said Nancy Keenan, who will soon be stepping down after eight years as president of NARAL Pro-Choice America.

“But there is an intensity gap — they don’t act on those values,” Keenan said. “The other side votes their anti-choice, pro-life values — it’s at the top of their political activity.”

She drew a contrast with the push for same-sex marriage.

“With marriage equality, gays and lesbians are fighting for something they didn’t have,” Keenan said. “In the case of reproductive rights, you’re trying to maintain the status quo. The millennial generation doesn’t see it as threatened.”

Another difference: the campaign for same-sex marriage has benefited greatly from personal testimony by gay couples, speaking out in legislative hearings and campaign videos. By contrast, although millions of American women have had abortions, relatively few speak out publicly to defend their decisions.

“If you know some women, you know a woman who’s had abortion,” said Dr. Anne Davis, who is medical director for Physicians for Reproductive Choice and Health and provides abortions as part of her practice in New York City.

“But you do not see women talking about their abortions,” Davis said. “They do what they need to do and move on. I can’t blame people for that.”

Davis, who learned abortion techniques during her residency at the University of Washington in the mid-’90s, said the procedure has become increasingly safe — notably with the advent of abortions via medication. She expressed dismay at the spate of restrictive laws that she and many of her fellow physicians view as ill-founded.

“Initially, we’d say, ‘That’s ridiculous’ — and now we’re stuck with them,” she said.

Despite all the furor, abortion has been commonplace in the post-Roe era, with about one-third of adult women estimated to have had at least one in their lifetime.

Of the roughly 1.2 million U.S. women who have abortions each year, half are 25 or older, about 18 percent are teens, and the rest are 20-24. About 60 percent have given birth to least one child prior to getting an abortion. A disproportionately high number are black or Hispanic; and regardless of race, high abortion rates are linked to economic hard times.

The Roe opinion, written by Justice Harry Blackmun, asserted that the right to privacy extended to a women’s decision on whether to end a pregnancy. States have been allowed to restrict abortion access at late stages of pregnancy, but only if they make exceptions for protecting the mother’s health — and the net result has been one of the most liberal abortion policies in the world.

At the time of Roe v. Wade, abortion was legal on request in four states, allowed under limited circumstances in about 16 others, and outlawed under nearly all circumstances in the other states, including Texas, where the Roe case originated.

One of the most liberal members of the current Supreme Court, Justice Ruth Bader Ginsburg, is among those who have questioned the timing of the Roe ruling and suggested that it contributed to the ongoing bitter debate.

“It’s not that the judgment was wrong, but it moved too far too fast,” Ginsburg said at Columbia University last year.

She said the court could have put off dealing with abortion while the state-by-state process evolved or it could have struck down just the Texas law, which allowed abortions only to save a mother’s life.

Asked about Ginsburg’s musings, Cecile Richards of Planned Parenthood said the Roe ruling was critically needed to curb unsafe abortions in states where the procedure was outlawed.

“Women were paying the price with their lives,” she said.

However, Carter Snead, a Notre Dame law professor who has studied abortion and bioethics, said Blackmun’s opinion was wrong to dismantle state anti-abortion laws so sweepingly.

“One key virtue of democracy is that, win or lose, the outcomes are generally seen as legitimate because all of the competing sides have had their say,” Snead said in an e-mail. “In Roe, the court short-circuited this process entirely, and handed a near total victory to one side of a bitterly contested question on the gravest of matters.”

Snead said abortion opponents have an enduringly compelling argument — “that the smallest, weakest, and most unwanted nevertheless have a claim on us.” But he said this argument can’t be translated into public policy without a change in the Supreme Court’s makeup.

Looking ahead, there’s no clear path toward an easing of the debate. Some activists and politicians say common ground could be found in a broad new campaign to curtail unintended pregnancies, but many anti-abortion leaders have shown little interest in this.

Some abortion opponents, such as Serrin Foster of Feminists for Life, urge bipartisan efforts to support pregnant young women as they pursue careers or education, so they don’t feel financial pressure to have an abortion. But supporters of legal access to abortion look askance at such proposals if they are coupled with calls to take abortion decision-making out of a woman’s hands.

For Carrie Gordon Earll, now senior policy analyst for the conservative ministry Focus on the Family, that Roe-established freedom of choice once seemed logical. She got pregnant in 1981 while attending a Christian college and opted to have an abortion.

She recently made a video expressing her regrets.

“I can look back at those 40 years and say without a doubt, the world is not a better place because of abortion, women are not in a better place,” she says. “What it has created is a world where you’re almost expected to abort if you’re pregnant at an inopportune time.”

In an interview, Earll mused on how the anti-abortion movement has persevered since Roe.

“We’ve had 40 years of marketing by Hollywood and the cultural elites that abortion is a good thing, and we still have a battle going on,” she said. “We’re holding our own.”

A similar refrain of perseverance is sounded by Dr. Douglas Laube of Madison, Wis., who began performing abortions as part of his practice a year after the Roe decision.

“It was important for women to be able to legally ensure their right to make their own decision,” said Laube, who is chairman of Physicians for Reproductive Health Choice. “But it served to polarize society politically.”

Laube is worried by the spread of anti-abortion state laws, but encouraged by the surge of women becoming obstetrician-gynecologists — a trend he hopes will ease the shortage of abortion providers.

“I see the movement toward the religious right being countered by a growing movement among practitioners and advocates for maintaining this as legal,” he said. “That means the controversy will continue. But it also means we will hold our ground.”