Newsweek’s Sarah Kliff nicely summarizes why abortion could be THE issue that stops ObamaCare.  I’ve made a similar argument in a paper on ObamaCare’s individual and employer mandates.

Two factors seem most salient:

1. One side must lose. ObamaCare would so infuse federal money into private insurance markets that either (A) taxpayers will be forced to pay for elective abortions, which would be unacceptable to pro-life Democrats, or (B) the restrictions necessary to prevent taxpayer funding would curtail access to private abortion coverage – even for women who don’t receive federal subsidies – which would be unacceptable to pro-choice Democrats.  Abortion is not one of those issues where opposing sides can meet in the middle.  There’s no way to, ahem, split the baby.
2. Abortion may be the one issue that Democrats care about more than health care. Democrats may therefore prefer to let ObamaCare die than violate their principles on abortion.  One can imagine pro-life Democrats saying, Health reform, yes – but not at the expense of the unborn, just as one can imagine pro-choice Democrats saying, Health reform yes – but not at the expense of a woman’s right to choose.

No matter which way ObamaCare comes down on abortion, the legislation could lose enough House Democrats to fall short of the 218 votes needed to win.