Republicans, led by Donald Trump, are in a headlong retreat on abortion. The latest evidence came this weekend, when Senator J. D. Vance said that Trump would veto a national ban on abortion were Congress to enact one, and implied that he would veto even a ban on abortions late in pregnancy. Before that, Trump said his presidency would be “great for women and their reproductive rights”: the latter phrase being generally used to describe an expansive right to abortion. And before that, Trump...