Donald Trump has discovered there is one issue even he cannot fool with. The bloviating billionaire asserted this week that in a Trump presidency there would be "punishment" for women who choose to have abortions.
It was a classic move. Espousing extreme solutions to key policy issues is his trademark: America should kill the families of jihadists (foreign policy); it should torture prisoners (national security); ban Muslims from entering; and build a wall on its southern border (immigration). His tactics have helped.
Not so with abortion. Barely had the words left his mouth before Trump faced a firestorm of criticism. Democrats, Republicans, pro-choice and pro-life groups launched attacks so fierce that, like a wounded animal, Trump was forced to retreat.
Abortion is the issue in America that can bring neighbours to blows. A Texas case now being heard has the power, potentially, to nullify the breakthrough 1973 Roe v Wade decision that struck down abortion bans. Many states put up legal barriers that hamper an abortion clinic's ability to operate and a woman's access to it.
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.