"I will never back down in defending a woman's right to choose," the likely Democratic nominee said, drawing a sharp contrast with his GOP rival.

"That's what's at stake," Obama added as he campaigned with his half-sister and Hillary Rodham Clinton, the pioneering former first lady he toppled during the Democratic presidential primary, at a "Women for Obama" breakfast fundraiser.

Obama packed his day with female-focused events in New York and Virginia, a reminder of his need to win over women who include some still smarting from Clinton's loss. She had tried to become the first woman to win the White House, and women were her base voters. They took her defeat hard, so much so that even a few are promising to vote for McCain.

Thus, to underscore his differences with McCain on women's issues, Obama cited Senate legislation from the spring that sought to counteract a Supreme Court decision limiting how long workers can wait before suing for pay discrimination.