Among all the disputes in this year’s presidential race — Iraq, the economy, health care, global warming — one issue stands out as clearly decisive: abortion rights. For the first time, Roe v. Wade, the 1973 Supreme Court decision that guaranteed the right to choose abortion, faces possible overthrow. If John McCain wins, it will in all probability go down. If Barack Obama wins, it will remain the law of the land. The high court has been moving to the right, and one more conservative judge could be enough to overrule Roe.

Both nominees are fully committed on opposite sides of the issue. Sen. McCain, who briefly seemed to support Roe eight years ago, now calls it “a flawed decision that must be overturned.” In 1999, he said he would not support repeal of Roe v. Wade because it would force “women in America to [undergo] illegal and dangerous operations.” Now he has the backing of the anti-choice lobby and a vice presidential running-mate, Sarah Palin, who describes herself as “pro-life.”

Sen. McCain says that overturning Roe will be “only one step in the long path toward ending abortion.” He calls for courage by pregnant mothers and compassion toward her and her newborn baby.

Sen. Obama’s official campaign Web site says: “Barack Obama understands that abortion is a divisive issue, and respects those who disagree with him. However, he has been a consistent champion of reproductive choice and will make preserving women’s rights under Roe v. Wade a priority as president. He opposes any constitutional amendment to overturn the Supreme Court’s decision in that case.” He has been criticized for having voted “present” instead of “no” on anti-abortion proposals as an Illinois senator, but Planned Parenthood said at the time that he was following its successful strategy to defeat the measures. Like the majority opinion in Roe, Sen. Obama has declined to speculate whether human life begins at conception, birth, or at some point in between.

The two platforms differ sharply on related issues. This year’s Republican plank supports a “human life” amendment to the Constitution, endorses “legislation to make clear that the Fourteenth Amendment’s protections apply to unborn children,” opposes the use of public funds for abortion, and favors judges “who respect family values and the sanctity and dignity of innocent human life.”

The Democratic campaign supports “access to affordable family planning services and comprehensive age-appropriate sex education,” to “reduce the number of unintended pregnancies and thereby also reduce the need for abortions.”

Don’t expect a full debate on Roe before the Nov. 4 election. Both nominees know how sensitive and inflammatory this issue is.

But pressure groups on both sides will certainly go to it. “Pro-lifers” will cite horrors of present legal abortions. Pro-choice advocates, as Sen. McCain once suggested, will warn about the horrors of illegal, inexpert, or self-inflicted abortion if Roe is overturned and states can once more outlaw abortion.