Opponents of abortion rights won a significant political victory last night, making it more likely that millions of American women will no longer be able to purchase insurance that covers abortion services.

At issue is what happens inside the new insurance exchanges, through which small businesses and people purchasing coverage on their own would shop for insurance. People purchasing coverage through the exchanges would be eligible for subsidies if their household incomes were below four times the poverty level. Abortion rights opponents don't want those subsidies going towards policies that cover abortion services, since that would mean taxpayers opposed to abortion were, in effect, paying for the procedure.

While such "mixing" of funds happens all the time, as Time's Amy Sullivan has observed, the Democratic leadership have tried to accommodate the opposition by proposing to create nominally distinct funds for abortion services or contracting out the financing to a private entity.

But the abortion rights opponents, led by Michigan Democrat Bart Stupak, wouldn't budge. They wanted the chance to introduce an amendment that would prohibit any plan that covers abortion services from accepting subsidies--a move that would effectively make policies covering abortion unavailable not only for people who need subsidies but, quite possibly, even those who don't.* After a furious day (and days) of negotiation, last night the Democratic leadership--desperate for every vote it could get--finally and very reluctantly gave in.

The amendment could very well pass, since those 40-odd Democrats will likely have the support of most Republicans. If it does, abortion rights advocates are hoping that they can prevail in the Senate.

It's not an unrealistic hope: As one lobbyist tells me, "The vote is better in the Senate"--not withstanding its generally more conservative tilt. But to prevail, abortion rights advocates will have to overcome the power of the U.S. Conference of Catholic Bishops, who have been lobbying furiously on this issue. Despite their avowed and long-standing support for measures that will help the poor--which health care reform surely will do--the Conference have urged members to vote against the House bill if the exchange provision doesn't change.

One interesting question: Is the White House weighing in on this? I asked spokesperson Linda Douglass if they were taking a position on Stupak's amendment--or, more generally, whether they had a position on whether policies inside the exchanges should cover abortion services. "We are not going to be commenting on provisions of the bill as they evolve today," she told me.

*Correction and clarification: I originally wrote that the Stupak amendment would prohibit any insurance plan inside the exchanges from covering abortion services. That's incorrect. The amendment would allow a plan to cover abortion services as long as it didn't enroll people who were using federal subsidies to help pay for premiums.

In theory, an insurer could offer two sets of plans, identical except for the fact that one covered abortion services and one didn't. The insurer would simply have to make sure the plans with abortion coverage were available only to people buying coverage without subsidies.

But would insurers actually go to that trouble? Abortion rights advocates are highly skeptical--and with good reason. Among other things, these advocates note, official projections suggest that just 15 percent of people in the exchanges--3 million of 20 million total--would be buying coverage without subsidies. That's probably not enough to make it profitable for insurers to offer such policies. As Laurie Rubiner, vice president for public policy and advocacy at Planned Parenthood, says, "While it doesn't appear to ban coverage of abortion in the exchange altogether, that is the effect."

Update: Amy Sullivan gives the backstory on this amendment. Her conclusion? The Democratic leadership could have avoided this episode by reaching out to abortion rights opponents earlier in the process.