Obama, Biden and Clinton did not explicitly name Trump, but their message was unmistakable. The three Democrats said, as much by their presence as their words, that these are unusual times for American democracy, that norms are being disregarded and extraordinary measures are required. The only living president who has not spoken out since Inauguration Day is Republican George W. Bush, though he has made little secret of his antipathy for Trump.

“Former presidents are uniquely qualified and situated to raise their voices and warn the American people if the country is taking a dangerous turn,” said Timothy Naftali, a historian at Columbia University’s School of International and Public Affairs. “Think of them as a sort of advisory council to the people of the United States. And when the advisory council sounds the alarm, the people should listen.”