I have written this piece for Slate. It begins:

President Donald Trump and First Lady Melania Trump have the coronavirus and the White House reported on Friday that the president is suffering from mild symptoms. With the president having just attended the debate earlier this week with Democratic presidential nominee Joe Biden, there could be concerns about Biden’s health as well. While Biden tested negative for COVID-19 on Friday, the lengthy incubation period means he’s still at risk.

Beyond wishing the president, first lady, and everyone who has contracted this terrible disease a full and speedy recovery, we need to ask as a matter of national importance what would happen if one of the presidential candidates died or became incapacitated before election day. Unfortunately, thanks once again to our Rube Goldberg machine for choosing the president, the answer to this question is somewhat murky and differs from state to state.

It concludes:

In a Friday update to his story, Pildes offered the following additional wrinkle: “If the RNC were deeply divided, and Republican electors then did not coalesce around a single replacement candidate, there might not be a majority winner in the electoral college. In that case, the House would choose the president from among the top three vote getters in the electoral college. In that process, each state delegation gets one vote.” In that case, one or two faithless electors deciding to support, say, Sen. Mitt Romney and allowing him to enter the top-three could potentially give us a candidate the people didn’t even see on the ballot. And because it could come down to the votes of House delegations, with each state getting one vote, we are going to see even more jockeying in places like Florida to see if Democrats can take over a majority of more House delegations.

In short, there would be a ton of uncertainty if we faced such a tragedy as a presidential candidate dying or becoming incapacitated during this period.