Mr. Trump chose on Wednesday morning to take Ms. Pelosi’s remarks as validation, never mind that she said that he was unfit for office even though she did not plan to seek to throw him out of power.

“I greatly appreciate Nancy Pelosi’s statement against impeachment, but everyone must remember the minor fact that I never did anything wrong, the Economy and Unemployment are the best ever, Military and Vets are great - and many other successes!” he wrote on Twitter. “How do you impeach a man who is considered by many to be the President with the most successful first two years in history, especially when he has done nothing wrong and impeachment is for ‘high crimes and misdemeanors’?”

In her comments to The Washington Post published this week, Ms. Pelosi did not entirely close the door, but she said that she was “not for impeachment” because it was so divisive and declared that the House should not head down that road unless there was “something so compelling and overwhelming and bipartisan.”

Many critics, especially on the left, argue that the first two of those standards have already been met or likely will be, but the third clearly has not, at least not yet. While Democrats could impeach Mr. Trump by themselves on a majority vote in the House, they would need at least 20 Republican senators to reach the two-thirds supermajority needed in the Senate to convict and remove him from office. Not even a single Republican senator has signaled openness to that at the moment.

Ms. Pelosi’s calculation reflects the lessons of history. No president has been removed or forced from office on a partisan basis. The only two presidents ever impeached, Andrew Johnson and Bill Clinton, were each put on trial by opposition Republicans only to be acquitted in the Senate. President Richard M. Nixon, on the other hand, resigned amid impeachment proceedings only after it became clear his fellow Republicans were abandoning him.