News that President and Mrs. Trump have tested positive for the coronavirus, announced by the President this morning, will no doubt be met by sneering and ill wishes by many of the President’s detractors. Yet it is being met by many more — millions of Americans, ourselves among them, and by millions abroad — with hopes and prayers for a speedy recovery.

This is a moment to reflect on the nature of our republic. Our Constitution marries a system of democracy with a compact among sovereign states. It grants the power to choose the president to the states. If they reach a majority in the electoral college, as they did in respect of Mr. Trump in 2016, that’s the end of it. Mr. Trump won that vote by a large margin.

If the states can’t deliver an electoral majority vote — a remote chance, but a chance this year — it then goes to the House. The members of the House are elected by districts apportioned by population. So it is sometimes called the people’s house. Then again, too, when the Representatives House acts to choose the president, the states hold sway, as the vote in the House is one per state.

We mention all this not out of fear that Mr. Trump mightn’t survive the coronavirus (the announcement today suggested he feels fine, though he’ll be in quarantine at the White House; the Times quotes sources saying he’s showing mild symptoms). Our purpose is just to underline the degree to which our popular and state sentiments are bound up in the presidency.

It is a moment to remember that the presidency is different from the legislative and the judicial branches. The executive is the only branch in which all the powers granted to it are assigned by the Constitution to a single individual. His duties may be carried out by various secretaries and envoys and camp aides. They serve, though, only at the President’s pleasure.

The Constitution does provide for the contingency that the president becomes incapacitated. The 25th Amendment, which was ratified in 1967, provides for, on the president’s signal, the temporary transfer of power to the vice president. Presidents Reagan and George W. Bush both did this when they were sedated for a medical procedure.

We’re nowhere near that yet, in the case of Mr. Trump, and we may never be. It’s the insidious nature of the coronavirus, though, that the disease can take sudden turns for the worse. Even without sudden adverse turns, it will likely keep the president cooped up in the White House during the crucial weeks where our country hurtles toward election day.

Finally, it’s a moment to remember how well Mr. Trump chose when he tapped Mike Pence to be vice president. The news reports have it that he has tested negative for the coronavirus. He’s a former governor and member of Congress, where he specialized in the budget. There’s no aspect of the executive branch on which he lacks for experience.

It may be that this development “upends” — to use a word being widely used in the press — the campaign. That, though, might be a calming effect, or even an elixir, in a campaign that has turned exceptionally bitter. Vice President Biden today paused from hurling insults at the President to wish him and the First Lady a speedy recovery. Welcome to the common ground.