But why is this topic so uniquely febrile in the United States?
Religion, you say. But the religious Right is actually losing strength. Last year America became a liberal standard bearer on gay marriage, legalising same-sex unions. And the statistics point to America becoming more secular. Even the fundamentalist Christian bloc is shrinking.
The real answer is in an increasingly partisan country, abortion is uniquely divisive. Abortion has always split America. It's no surprise, then, that a recent poll found that, for a record number of Americans, a politician's stance on abortion is the single most important factor in shaping how they vote. Trump should have known better.
His reaction is telling: humility, apology, backtrack - the kind of compromises that the career politicians he so derides make all the time.
This may be the first political U-turn for The Donald, but if he goes much further, it will certainly not be his last. And it shows why he may not be as iconoclastic - or invincible - as his passionate supporters believe.