... If Mr. Trump expected Barack Obama, who will be the first president since Woodrow Wilson to continue living in Washington, to retire to silence, he got a rude awakening on Wednesday. Mr. Obama told reporters at his last news conference that action by the new president on any number of things — voting rights, rescinding the so-called Dreamers program for immigrants who came to this country as children — “would merit me speaking out.”

Mr. Obama’s critics were quick to point out that this simply wasn’t done. Presidents are supposed to slip off the stage, take up hobbies and charities, let their successors have a go at it. But that hasn’t always been the case.

... Harry S. Truman, for example, spent several years in semiretirement, writing a two-volume defense of his almost eight years in the White House. But in 1960 he returned to politics. He despised Vice President Richard M. Nixon, but he detested John F. Kennedy’s father, Joseph, even more. The elder Kennedy was a strident critic of Franklin D. Roosevelt, and Truman joined Eleanor Roosevelt in raising questions about the younger Kennedy’s youth and inexperience. Though Roosevelt eventually warmed to Kennedy, Truman never did.

Then there’s Theodore Roosevelt, who already had his share and more of hobbies and adventures to tend to in 1909, but nevertheless spent his early post-White House years lambasting his successor, William Howard Taft. Roosevelt was so incensed by Taft’s coziness with big business and his apparent lack of interest in his predecessor’s progressive agenda that he ran for the presidency in 1912 as a third-party candidate.

All of these ex-presidents made their choices under fairly conventional circumstances — what Mr. Obama called “normal back-and-forth, ebb-and-flow of policy.” Taft didn’t come into office promising to erase Roosevelt’s legacy.

But these are not conventional circumstances, which is why it’s not surprising that Mr. Obama has no intention of entirely retreating from the political ramparts. ...