The result would turn back the clock to the days before Roe v. Wade when abortion was legal only in some states, but not in others. There is every indication that about half the states would make abortion illegal within a year of Roe being struck down, according to the Guttmacher Institute. The Center for Reproductive Rights, which challenges abortion restrictions around the country, puts the number at 30 states. For one thing, abortion bans already on the books in some states would suddenly kick in. And some Republican-controlled state legislatures would outlaw abortion immediately.

Even with Roe and subsequent decisions upholding abortion rights, more than half the states have enacted barriers like mandatory waiting periods, “counseling” sessions lacking a real medical justification; parental consent or notification laws; and onerous clinic “safety” rules intended to drive clinics out of business.

Mr. Romney is a vocal supporter of this continuing drive in the states and in Congress to limit the constitutional right, even without overturning Roe. To a large degree, the anti-abortion forces have succeeded. In 1982, there were about 2,900 providers nationwide; as of 2008, there were less than 1,800. In 97 percent of the counties that are outside of metropolitan areas, there are no abortion providers at all.

We do not need to guess about the brutal consequences of overturning Roe. We know from our own country’s pre-Roe history and from the experience around the world. Women desperate to end a pregnancy would find a way to do so. Well-to-do women living in places where abortion is illegal would travel to other states where it is legal to obtain the procedure. Women lacking the resources would either be forced by the government and politicians to go through with an unwanted or risky pregnancy, attempt to self-abort or turn to an illegal — and potentially unsafe — provider for help. Women’s health, privacy and equality would suffer. Some women would die.

Mr. Romney knows this, or at least he used to. Running for the United States Senate in Massachusetts in 1994 against Edward Kennedy, Mr. Romney spoke of a young woman, a close relative, who died years before as result of complications from an illegal abortion to underscore his now-extinct support for Roe v. Wade. In a report in Salon last year, Justin Elliott, a reporter for ProPublica, found that when the young woman passed away, her parents requested that donations be made in her honor to Planned Parenthood. That’s the same invaluable family-planning group that Mr. Romney has pledged to defund once in the White House.