Tonight, on prime-time TV, Barack Obama will announce an executive action offering amnesty to a large number of illegal migrants—perhaps 5m. Presidents from both parties have used their authority in this area to shield people from deportation before, but Mr Obama’s action will be different in scale and in kind. In 1990 George H.W. Bush tweaked a 1986 immigration law to give an amnesty for the 1.5m children and spouses of people covered by that act. Mr Obama, by contrast, is acting because Congress has repeatedly failed to pass any legislation to deal with the 11.3m undocumented people living in America. The president’s aim is laudable, but America is not supposed to be ruled by fiat. In the short term his action will infuriate many Republicans: expect talk of government shutdowns and impeachment. Looking further ahead, it will set a worrying example for future occupants of the White House.