Yet they all lack historical perspective. In fact, the notion that the Obama presidency is all but over has arrived right on schedule for any second-term president. By this stage of their presidencies, Bill Clinton and George W. Bush, like Reagan before them, were all being written off as finished, not least of all by those who had been their strongest supporters.

However, history also demonstrates a larger truth that the commentators ignore today. Quite a few of the most significant achievements of the Reagan, Clinton and Bush presidencies took place in their final two years. The public may pay ever less attention in the final years, and the president’s perceived power may be on the wane. Yet he still possesses the same great constitutional authority.

Reagan’s most significant diplomacy with Mikhail S. Gorbachev, which contributed to the end of the Cold War, took place in his final two years. In Mr. Clinton’s final two years, he paved the way for China’s entry into the World Trade Organization, cementing the international economic order we have today, and began military action in Kosovo without obtaining approval by the United Nations Security Council, thus setting a precedent for similar American military actions since. Mr. Bush’s last two years saw the “surge” of American troops into Iraq, altering the course of the war there; the largest financial bailout in the nation’s history; and the signing of the agreement with Iraq to remove American forces at the end of 2011.

Mr. Obama’s presidency likewise has a long way to go: in the new war with the Islamic State, on a nuclear deal with Iran, on new trade agreements, on a new immigration policy, and in areas we can’t even imagine yet. So how can we explain the premature doomsaying?

If there is such a thing as a national psychology, then we might call this a kind of collective projection. We claim that a president is tired or looks tired, when what we really mean is that we are tired of him. (In fact, second-term presidents may be less tired than their graying hair and wrinkled faces suggest. They often play more golf than they did in their first terms.) By his sixth year in office, any president is ridiculously overexposed. We’ve seen him and heard him far too many times.