Last year, the U.S. Supreme Court voted in a 5-4 decision to uphold a federal ban on late term abortions, regardless of the health of the mother. The decision not only signaled a significant shift in the court’s approach to the controversial abortion issue, but revealed just how important the composition of the Supreme Court can be — and the extraordinary power the next president will have to control that.

The last time the court visited the abortion question, in 2000, it had struck down an almost identical Nebraska law — in part, because it didn’t account for the woman’s health. Causing those who procure abortions to “fear prosecution, conviction and imprisonment” was an undue burden on a woman’s right to choose an abortion, wrote Justice Stephen Breyer for the majority, in Stenberg v. Carhart.

Nothing had changed between then and the court’s 2007 ruling. Except, that is, the composition of the court.

In 2005, Justice Sandra Day O’Connor, who had consistently upheld the right to privacy that protected abortion ever since Roe v. Wade, retired. President George W. Bush had since appointed John Roberts as chief justice and Samuel Alito as associate justice.

Sen, John McCain, the Republican presidential nominee, has said repeatedly that if he’s the next president, he’ll appoint more justices like Alito and Roberts, given the chance. And the next president is widely expected to have that chance.

“John McCain’s appointment of the next justice could be the most significant appointment in half a century,” said David Cole, a law professor at Georgetown University, noting that the trend has been toward appointing younger justices who could be on the court for 30 or 40 years. “We would have a solid far-right conservative block for the next several decades. That cannot be undone by a subsequent election if people decide that the court is far to the right of them. There’s nothing we can do as citizens.”

Already, as Geoffrey Stone, a University of Chicago law professor, has observed, “The current makeup of the Supreme Court renders it the most conservative group of justices in living memory.”

Seven of the nine sitting justices were appointed by Republican presidents, and there are no true liberals on the court in the model of Thurgood Marshall, William Brennan Jr. or Earl Warren.

As Justice John Paul Stevens told The New York Times last fall, every judge that’s been nominated to the Supreme Court since 1971, including himself, “has been more conservative than his or her predecessor. Except maybe Justice Ginsburg. That’s bound to have an effect on the court.”

In her dissent in last year’s abortion case, Justice Ruth Bader Ginsburg called the decision “alarming.” She wrote:

“It tolerates, indeed applauds, federal intervention to ban nationwide a procedure found necessary and proper in certain cases by the American College of Obstetricians and Gynecologists (ACOG). It blurs the line, firmly drawn in Casey, between pre-viability and post-viability abortions. And, for the first time since Roe, the court blesses a prohibition with no exception safeguarding a woman’s health.”

That willingness to ignore precedent and retract settled rights established years before is something many court-watchers expect to see more of if McCain is elected president.

“A President McCain will likely have a much greater opportunity to move the Supreme Court than would a President Obama,” said Doug Kendall, president of the Constitutional Accountability Center, a legal watchdog group in Washington. “Simply because it is actuarially likely that the justices that retire during the next presidency will be the court’s more liberal justices. So a conservative president could move the court sharply to the right, while a progressive president will most likely be replacing existing progressive justices.”

At 88, Stevens is widely expected to retire within the next few years. Others have speculated that Ginsberg, 75, or even Justice David Souter, who has admitted he’s considered resigning, could depart as well. “McCain could finally achieve what conservative legal activists have been looking for for decades,” said Kendall, “which is a truly conservative, if not reactionary, Supreme Court.”

Because so many of the court’s recent rulings have been decided by a narrow 5-4 majority, the change in just one justice could yield a fundamental change in the enforcement of civil rights, the ability of the federal government to regulate safety and the environment, the breadth of executive power and the reach of habeas corpus.

In the 2006-2007 term, a remarkable one-third of the court’s cases were decided by 5-4 votes. Though the current term has yielded fewer 5-4 rulings, the justices have nonetheless still divided 5-4 on key issues like the rights of detainees at Guantanamo Bay. But the last few years’ worth of decisions suggest sharp divisions on a broad range of critical issues.

“Obviously the $64,000 question is Roe v. Wade,” said Edward Lazarus, an attorney who clerked for Supreme Court Justice Harry Blackmun and wrote, “Closed Chambers: The Rise, Fall, and Future of the Modern Supreme Court.” “They’ve been one vote away for a long time. Whether affirmative action would become completely unconstitutional would be another one.”

But there are less obvious issues as well. Take the case ofRapanos v. U.S., in which the Supreme Court by a 5-4 majority upheld the federal government’s right to protect wetlands and waterways under the Clean Water Act against a challenge by developers eager to build a shopping mall and condos. “If you replace Stevens with a more conservative member, you’ll have a greatly restricted Clean Water Act,” said Kendall.

Last year’s global-warming ruling, Massachusetts v. EPA, in which the court, again ruling 5-4, held that the federal government not only has authority to regulate the emissions of greenhouse gases, but must come up with a better reason for refusing to do so, could have easily come out the other way as well.

Then there are the executive power cases, where the court has reined in the Bush administration’s efforts to hold “enemy combatants” indefinitely at Guantanamo Bay without any rights to challenge their detention.

“In context of the so-called ‘war on terror’ the court has insisted that the president does not have unchecked power but must operate within the rule of law and subject to judicial review,” said Cole. “They did that in the Boumedienne decision this past summer, and in the Hamdan decision last summer. But both of those were 5-4 decisions. One more justice replacing one justice in the 5-4 majority would give a majority to those justices who believe the president should have effectively unfettered power in the war on terror.”

“Each of those cases would almost certainly have been decided differently if Stevens had retired and were replaced by a justice like Roberts or Alito,” agreed Kendall. Those are precisely the sort of justices that McCain has pledged to appoint.

“John McCain has promised more of the Bush nominees,” said Judith Schaeffer, legal director of People for the American Way, a nonpartisan organization dedicated to defending fundamental American values. “He’s already said he’d give America more Roberts’s, more Scalia’s, more Thomases. It would be more of the same. But it would mean a court even farther to the right than where it is now.”

Of course, judges don’t always rule along political lines. But in a recent study by University of Chicago law professor Cass Sunstein that hewrote about in The Washington Independent, Sunstein found partisan voting pervasive in the federal courts. “Republican appointees vote to uphold liberal agency decisions at a significantly lower rate than conservative agency decisions. Democratic appointees vote to uphold liberal agency decisions at a significantly higher rate than conservative agency decisions.” What’s more, contrary to conservatives’ complaints of “judicial activism” by liberal judges, in fact, conservative members of the Supreme Court showed the most “activist” voting patterns – that is, they’re the ones who most often vote to strike down a federal or state law.

The federal courts are already weighted heavily with Republican nominees. On the federal appellate courts, for example, “Republican-nominated judges out-number those nominated by a Democratic president 98 to 67,” People for the American Way has found. Meanwhile, 9 of the 13 circuits have a majority of Republican nominees.

To be sure, how any particular judges will rule, and what issues will come before them, is difficult to predict. “Who knew we’d have a war on terror that made executive power so prominent an issue?” asked Barry Friedman, a law professor at New York University. “Who knows what new technologies may arise that will raise new issues of individual liberties?”

On the other hand, “If McCain gets the job and he appoints conservative judges, we’re going to have a real hard conservative majority on the Supreme Court for the first time in ages,” said Friedman. “There’s always been someone who played to the middle. If you add one more Scalia or Thomas or Roberts or Alito to this bench, you’ve got a very hard 5-person conservative majority.”

While it’s possible that McCain could change his mind about the kind of justices he would appoint, his choice of Sarah Palin as the vice presidential nominee is telling. “It indicates an intense desire to please and motivate his political base,” said Lazarus. “Historically a lot of presidents on the Republican side have used Supreme Court justices to placate their conservative base even if they themselves aren’t that conservative,” he added, noting the appointment of Clarence Thomas by the first President George Bush.

“What that base cares about is the Supreme Court,” adds Friedman. “Issues like reproductive rights, religion in schools, religion in public life. A lot of what they care about they can’t do any other way.”

McCain’s statements about the Supreme Court also suggest he’s eager to use his power over the court to win over the right wing of his party. Indeed, in June, the former Vietnam prisoner of war called the court’s ruling that Guantanamo detainees have a right to challenge their indefinite detention without charge “one of the worst decisions in the history of this country.”

A solid conservative majority on the court will encourage conservatives to try to get their issues heard. “If the Supreme Court solidifies into a conservative institution there will be a much more concerted effort by litigants and sympathetic state and local officials to frame cases that bring those issues to the court’s attention,” said Kendall.

And the court then would be far more likely to respond than it is now. “If a conservative majority solidifies on the Supreme Court you’re likely to see them reach out to hear cases that will give them the opportunity to overrule some of the precedents in areas such as civil rights and abortion,” said Kendall.

Having even one more conservative justice on the court would give the conservatives that majority. And as the conservatives on the Supreme Count demonstrated last year in the so-called “partial birth abortion” case, they’re not hesitant to seize such opportunities when they arise.