Blaming a predecessor has been done before, of course. Obama criticized his Republican predecessor George W. Bush for throwing the economy into a ditch and getting the United States into a "dumb" war in Iraq. Earlier, the Republicans had delighted in mocking Democrat Jimmy Carter for being weak and out of touch on both domestic and foreign policy. And Democrats for many years pilloried Republican Herbert Hoover, Democratic President Franklin Roosevelt's predecessor, for a poor response to the Depression.

But with Trump the hostility seems to be more personal, sustained and harsh. Perhaps he attacks Obama to energize die-hard conservatives who hate Obama and condemn his policies as too left-wing. The problem is that former presidents can be helpful to incumbents if they are on good terms, and Trump seems to be cutting off this possibility. Former President Richard Nixon, though condemned by Democrats, secretly advised Bill Clinton on foreign policy, and Clinton found the counsel to be very helpful. George H.W. Bush and Clinton helped to raise money for victims of natural disasters around the world at the request of President George W. Bush.

The point is that former presidents possess a wealth of knowledge and talent, and they have cultivated relationships with other leaders at home and abroad. All of this can be invaluable to an incumbent. But first they have to get along and develop mutual respect. That hasn't happened between Trump and Obama, and it doesn't seem likely any time soon.