I would venture a guess than many, if not most, of the pro-choice advocates were supporters of Hillary Clinton's candidacy for President in the primary. At least that was my experience among my friends and colleagues. A good number of those women have made the transition to Obama at this point in time.
However, after listening to the debate last night, I would expect many (if not most) of the hold-outs are there now. That is, those last few hangers-on (the fence) most certainly hung up on John McCain.
John McCain decided to help them decide last night during the discussion of the abortion issue. The truth of the matter is that during the primary, some pro-choice groups felt that Barack Obama's position on choice wasn't strong enough, see Obama and the Acceptable Abortion, even though now the GOP tries to paint him an abortionist and "baby killer." McCain Repeats Debunked ‘Born Alive’ Attacks in Debate.
However, McCain's extreme anti-choice reaction occurred during Obama's discussion of the issue, as noted by Steve Benen of The Washington Monthly:
McCain had tried to characterize Obama's position on abortion as extreme, and Obama explained that he'd support restrictions on late-term abortions if proposals included provisions to protect the life and health of the mother.As I watched him speak the words, I couldn't believe it. Jason Linkins of Huffington Post, McCain Mockingly Suggests That Concerns For A Mother's Health Are Extreme, describes it:
The differences of opinion surrounding the issue of access to safe and legal abortion, and a woman's right to choose to have one, have long been a mainstay of political debate. But tonight, I believe, featured a historical moment in that debate, because until tonight, I had never seen the matter of a woman's health given AIR SCARE QUOTES. But that's precisely what John McCain did, sneeringly, as he attempted to portray support for a mother's health as an extreme position, when in fact, it is a mainstream position -- ground that even fervent pro-life individuals often concede. (Emphasis added)Watch this & tell me he doesn't want to lose: