One of the banes of American politics in the past 25 years or so has been the domination of ideology over pragmatism in political discourse. Sometimes it is invigorating: only in America do you still have a real, lively, evenly matched debate about whether gays are inferior to straights, whether the death penalty is a moral necessity, or whether embryonic stem-cell research is an abomination.

But for people caught in the middle — gay couples wanting some legal protections short of marriage, people with Parkinson’s needing scientific stem-cell research to save their own lives, or women dealing with an unexpected pregnancy — the polarisation offers no relief.

The safety valve is federalism, the constitutional system that allows different states to have different laws. And so in Massachusetts gay couples enjoy real equality under law — better than the British civil partnership compromise — while in Virginia they have reduced rights even to enter into private contracts. And President Bush can prevent federal funding of embryonic stem-cell research, while California governor Arnold Schwarzenegger can kick-start a massive programme designed to do exactly what the president abhors.

This is the way it is supposed to work, and in a country that includes both San Francisco and Salt Lake City, Greenwich Village and Colorado Springs, it makes a lot of sense. In the 1960s and early 1970s abortion laws were similarly diverse, with most states moving towards a European-style compromise — allowing for legal first-trimester abortions with an array of restrictions on others.

Then the Supreme Court stepped in, and in one of the more far-reaching cases in its history, Roe v Wade, ruled to designate abortion a constitutional right. And so, overnight, every state had to comply with the most liberal abortion regime imaginable. Even preventing the near-infanticide of late-term, partial-birth abortions became impossible. And the conservative Christian response to this piece of judicial overreach was critical to the emergence of what is now the religious right.

But last week came the glimmers of the end of that era. Despite two new justices on the Supreme Court who are supposed to be hostile to Roe, the court still has no secure majority for overturning it. But it’s getting closer and closer. The religious right, moreover, is in no mood to wait. So South Dakota became the first state to pass and have the governor sign a law that represents the full ambition of the pro-life movement. This is the moment they have long waited for.

And it’s the moment when their ideology may come undone. For decades the pro-life movement has railed against both Roe and activist courts and upped the rhetorical ante so that even the morning-after pill is regarded as tantamount to murder. Because the court ensured that pro-lifers’ principles would never be put into practice, they could entertain purist ideologies to their hearts’ content. But now they actually have to begin to deal with the practical implications of recriminalising abortion. And their first step has been instructive.

The South Dakota bill is Roe in reverse. It criminalises all abortions at any stage after conception. There are no exceptions even for rape, incest or the health of the mother. Even where the mother’s life may be at stake, the law demands that doctors “make reasonable medical efforts under the circumstances to preserve both the life of the mother and the life of her unborn child”. The penalty is five years in prison for the doctor.

The shrewder types in the pro-life movement don’t want this law passed right now. While they agree with the law in substance, they fear that the Supreme Court is not quite ready to legitimise such a drastic step. They worry that the law will get struck down swiftly and firmly and end up helping support Roe, scare the American centre and put back their cause.

The trouble they face, however, is a profound one. When you have spent the past couple of decades arguing that the abortion of a day-old zygote is morally indistinguishable from the killing of a 10-year-old child, you have essentially rejected any possibility of a compromise.

For the Republican party the dilemma is particularly acute. Karl Rove has reconstructed the party so that its core membership is that of evangelical Christians who believe politics should be governed by religious principles. These people can brook no compromise on the abortion issue and the gay issue. So they want a total ban on all abortions, and a constitutional ban on any legal protections for gay couples.

Most Americans, however, fwant something far less drastic. A large majority favours either civil unions or civil marriage for gay couples. And a big majority wants more restrictive abortion laws but not an outright ban — let alone something as draconian as South Dakota’s. In the past Karl Rove could pander to the religious base, knowing that the Supreme Court would never allow the issue to actually matter, or the laws to actually change.

He has now been hoisted on a faith-based petard. After South Dakota, the debate is transformed from an abstract discussion of whether you’re for “life” or for “choice” into a series of very practical questions. Should a doctor be prosecuted for first-degree or second-degree murder? Should the mother be prosecuted as well? How can we enforce an exception for rape or incest when we only have the word of a woman to that effect?

That debate itself galvanises the pro-choice Democratic voter, and freaks out the moderate Republicans as well. Gay marriage bans only ever affected a small minority, so it was hard for a backlash to the backlash to gain ground. But not on the abortion issue.

The lesson is an obvious one: be careful what you wish for. But the good news is that Americans will now have to abandon ideology for real politics: what can be done? How do we practically lower the abortion rate? How do we enforce abortion bans of varying degrees? That debate was prematurely ended three decades ago. The religious right and the Republican party may well regret that it is now poised to resume.

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