Who's Patrick Leahy? We'll get back to that later. First, we have to take a look at the current situation in the US, where the news is obviously dominated by the same disease that forces most people in Europe to live behind closed doors by now.
To prevent chasing away potential readers with an overdose of judicial lingo, I'll sketch out in a few short steps what might happen if this pandemic keeps spreading across all fifty states.
Step 1: The number of infections spirals out of control and the health care system can't cope with the problem. In spite of its late start, the American government takes various measures, including a ban on large gatherings.
Step 2: The president of the US is forced to postpone the upcoming elections. Apart from the fact the turnout might be extremely low, he can't force all those officials who man the desks to sit there in the presence of potential patients all day.
Step 3: On the 20th of January 2021, Donald Trump's mandate ends. Since there haven't been any elections, he's no longer the president from that day onwards. Normally, the vice-presicent would take over, but his mandate is obviously coming to an end as well. With both seats vacant, the speaker of the House of Representatives would become president ad interim until new elections can be organised. That would be Nancy Pelosi, but there is a problem.
Short clarification: In November 2020 the presidential elections aren't the only elections in the US. The Americans will also elect a part of the Senate and the House of Representatives. The American elections aren't identical to the proceedings known in most of Europe. The American Senate is divided in three so-called classes, class 1, class 2 and, yes, you could have guessed it by now, class 3. Every two years there are elections for one of these three classes. This year, it's all about the 33 seats in class 2, with added elections in Arizona (to replace the deceased John McCain) and in Georgia (where Johnny Isakson has resigned due to health reasons). In the House of Representatives, all seats are up for grabs.
Step 4: If the elections are postponed, the mandate of Nancy Pelosi will also end on January 20th 2021. Luckily, the American constitution contains a solution for this problem. In that case, the senator with the longest service record of the largest party will become the temporary new president.
Step 5: If the elections are postponed, the Senate will become a lot smaller, because all senators in class 2 will lose their seat. In that case 23 republicans and 12 democrats will have to leave, meaning the democrats will suddenly have the majority in the Senate.
Step 6: The repulicans will try to fill these lost seats with new senators, but that might turn out to be problematic. The congresses of the individual states where senators in class 2 are up for (re-)election can decide to nominate a candidate, but the governor of that state can veto this decision, leaving the seat vacant until real elections are held. By coincidence, most Senate seats in class 2 that could be filled this way are found in states with a democratic governor.
Step 7: As the democrats have become the largest faction in the Senate and the republicans are unable to regain their majority at state level, the democratic senator with the longest service record all of a sudden finds himself in the position of head of state. And that's a senator from Vermont. Not Bernie Sanders, but the aforementioned Patrick Leahy .
It would be quite ironic to see the White House fall, without any form of democratic elections, into the lap of a senator who has spoken out in favour of abortion, against the death penalty, in favour of the legalisation of cannabis, against the discriminations of LGBTQ's, in favour of higher taxes for the rich and against the war in Iraq. It would also be a better result we can expect if the democratic party really nominates Joe Biden.
: Leahy is not very known outside the US and a lot of people in the US haven't heard of him either. Luckily, Wikipedia comes to the rescue: https://en.wikipedia.org/wiki/Patrick_Leahy