While neither candidate is anticipating reviving a draft any time soon, they disagree on one key component: whether to require women to register for Selective Service. Obama supports a requirement for both men and women to register for the draft while McCain doesn't. The Selective Service system was revived by Jimmy Carter in 1980 to require all men between the ages of 18 and 25 to register, just in case a massive expansion of the military is necessary.
Keeping women out of the Selective Service has long been controversial among women's rights groups as well as men who've faced penalties for not registering. While Congress and the Supreme Court have exempted women using the military's combat rules, Obama's national security spokeswoman says, "Women are already serving in combat [in Iraq and Afghanistan] and the current policy should be updated to reflect realities on the ground. Barack Obama would consult with military commanders to review the constraints that remain."
McCain is in favor of the current Department of Defense restrictions on women in combat units. A campaign spokesman says of his position on the Selective Service, "Sen. McCain strongly believes that an all-volunteer force is preferable to a conscripted force. The tools available to recruiters have historically enabled the all-volunteer force to attract sufficient numbers of qualified recruits."