But the two presidential candidates disagree on a key foundation of any future draft: Mr. Obama supports a requirement for both men and women to register with the Selective Service, while Mr. McCain doesn't think women should have to register.

Also, Mr. Obama would consider officially opening combat positions to women. Mr. McCain would not.

"Women are already serving in combat [in Iraq and Afghanistan] and the current policy should be updated to reflect realities on the ground," said Wendy Morigi, Mr. Obama's national security spokeswoman. "Barack Obama would consult with military commanders to review the constraints that remain."

According to his campaign, Mr. McCain supports the current Department of Defense restrictions on women in combat units, including armor, field artillery and special forces.

In 1980, President Jimmy Carter revived the Selective Service system, which compiles a list of nearly all men in the U.S. between 18 and 25 in case a crisis forces the government to undertake a massive expansion of the military.

Both Congress and the Supreme Court have exempted women from registration because of the combat rules.

For years, that position has rankled some women's rights groups and men who face penalties for not registering -- including loss of employment with the federal government -- at a time when female soldiers regularly find themselves in dangerous situations in Iraq and Afghanistan, both conflicts without defined battlefields.

Mr. McCain, a decorated former Navy pilot who spent five years as a prisoner of war in Vietnam, speaks often of how his military service has helped prepare him for the role of commander in chief and how his time as a captive in Hanoi reinforced his love of country.

Yet he doesn't want to see a return to mandatory service, for men or women, according to his presidential campaign.

"Sen. McCain strongly believes that an all-volunteer force is preferable to a conscripted force," said Paul Lindsay, a spokesman for the campaign. "The tools available to recruiters have historically enabled the all-volunteer force to attract sufficient numbers of qualified recruits."

His views are echoed by many high-ranking officers in the military, who prefer a force of motivated volunteers. But some of the same officers have also expressed concerns about the strains of more than six years of sustained combat in Afghanistan and Iraq, especially if the U.S. commitment in both countries doesn't end in the near future.

Mr. Obama has said repeatedly that he will draw down the U.S. military presence in Iraq if he becomes president, but he has also said he would increase the number of troops in Afghanistan, where Taliban forces have seen a resurgence in recent years.

During a CNN/YouTube debate for Democratic presidential candidates last year, he said he doesn't "agree" with the draft.

But he did say women should be expected to register with the Selective Service, comparing the role of women to black soldiers and airmen who served during World War II, when the armed forces were still segregated.

"There was a time when African-Americans weren't allowed to serve in combat," Mr. Obama said. "And yet, when they did, not only did they perform brilliantly, but what also happened is they helped to change America, and they helped to underscore that we're equal.

"And I think that if women are registered for service -- not necessarily in combat roles, and I don't agree with the draft -- I think it will help to send a message to my two daughters that they've got obligations to this great country as well as boys do."

Elaine Donnelly, a former member of President Bill Clinton's Commission on the Assignment of Women in the Armed Forces, dismissed Mr. Obama's comparison of the roles of women and black soldiers, arguing that males and females, in general, aren't equal on the battlefield.

"There are differences between men and women where physical strength is an issue," said Ms. Donnelly, who heads the nonpartisan Center for Military Readiness. "There are a lot of civilian feminists who are making unreasonable demands on the military."

Nancy Duff Campbell, co-president of the National Women's Law Center, argues that women should have a chance to compete for any position in the armed forces.

"I hope a new president will revisit the restrictions," she said.