WHEN PRESIDENT BUSH signed a ban on partial-birth abortion last week, it marked the first congressional rollback of Roe v. Wade since the landmark Supreme Court decision legalizing abortion was handed down in 1973. And it marked the success of an idea as well. The idea, of course, is that abortion is inhuman and should be limited as sharply as possible and ultimately outlawed altogether.

However, it takes more than an idea to enact a law. Yesterday's rollback--indeed the rise of the pro-life movement across the country--would not have occurred except for one thing. That was the embrace by conservatives of the antiabortion cause and the belated conversion of one conservative in particular, Ronald Reagan. Once this happened, opposition to abortion became a top priority of conservatism's chief political vehicle, the Republican Party. Now, says Republican Congressman Henry Hyde, "it's the issue that won't go away."

Not in America anyway. Outside the United States, serious opposition to abortion is rare. Conservatives world-wide tend to agree on limited government, low taxes, respect for traditional values and strong law enforcement. But a commitment to protecting unborn children is unique to American conservatism.

Even here, full-throated conservative opposition to abortion is a relatively recent phenomenon. By the time Roe v. Wade was decided 30 years ago, 18 states had already liberalized their abortion laws. The opposition came mostly from the Catholic Church and assorted Protestant evangelicals, not from conservative leaders. There was no national campaign, as there is today, to rally the pro-life forces, stage marches, pressure

THE MOST TELLING EXAMPLE of conservative indifference to the abortion issue occurred in California. In 1967, then-Gov. Ronald Reagan signed a bill that virtually decriminalized abortion. At the time, Reagan was troubled by the passionate lobbying against the bill by Cardinal Francis McIntyre. But on the advice of two of his most conservatives advisers, Ed Meese and Lyn Nofziger, Reagan signed anyway. He persuaded himself that the measure would have little impact. Instead, it prompted a surge in abortions.

Roe v. Wade changed the terms of the abortion debate, but not instantly. At first, conservatives were more upset by the decision's dubious legal reasoning and its creation of a new constitutional right unmentioned in the Constitution itself than by the actual impact. But it soon became clear that the supposedly complicated three-trimester scheme laid out in the ruling wasn't really so complicated. It meant abortion on demand, and the number of abortions soared into the millions.

Roe v. Wade had moved America into a dark new world. Defending the decision, radical feminists insisted that an unborn child was no more valuable as human life than a wart. A lucrative abortion industry grew up. The Democratic party endorsed an unfettered right to an abortion in its 1980 platform.

Messrs. Reagan and Hyde were among the first Republicans to have strong misgivings. Within a year after signing the abortion bill, Reagan told political writer Lou Cannon that he'd never have done so if he'd been more experienced in office. It was "the only time as governor or president that Reagan acknowledged a mistake on major legislation," Cannon writes in his new book, "Governor Reagan: His Rise to Power." By 1980, Reagan was campaigning for president in favor of banning abortion in all but rare cases.