The way us presidents are elected has changed significantly over the two centuries since the 1787 constitution provided only a bare outline on the process of choosing an american national executive.

Indeed, controversy started when the process was scarcely a decade old: thomas jefferson and his vice-presidential running mate aaron burr received the same number of electoral votes in the 1800 election, forcing an amendment to make the process simpler.

President donald trump is experiencing only mild symptoms after contracting coronavirus, and no main partys presidential candidate has ever pulled out of the race before election day because of illness. one vice-presidential candidate passed away shortly before the 1912 poll and was not replaced on the ballot. here we look at the rules that would come into play if a candidate were too sick to continue running.

The candidates political party has to make a choice. it is not a constitutional matter but a question for republicans and democrats, and both parties have a clear set of rules setting out how to handle such a situation. for the republicans, the 168 members of the republican national committee which comprises senior party officials from each state would vote to choose a new presidential candidate. the rnc is under no obligation to elevate a vice-presidential nominee, though it would be the obvious choice.

The anointment of a new candidate would bring a host of logistical and legal complications. state deadlines for candidates to register on the ballot have passed, and millions of ballots have already been printed, mailed out, or indeed returned by voters.

That means a change in candidate at this late date means it would probably be impossible to swap names on most ballots. but the arcane us electoral college system might mean that any replacement selected by party officials could yet stand in his place.

Each state chooses a group of electors, who are then expected to vote in the electoral college for the candidate who won the popular vote in their state.

Many states have laws requiring electors to obey or promise to obey the outcome of the popular vote, but it is possible they would still vote for the replacement candidate in places where the ailing candidate wins.

It is hard to imagine they would be sanctioned for violating these laws; in any event, the sanctions are so mild no elector would be deterred by them in this situation, wrote richard pildes, an election law expert at new york university school of law, in the washington post.

The supreme court this year ruled that electors could be punished for failing to vote for the candidate who won the popular vote in their state, but explicitly left often the possibility that electors could vote differently if the candidate was deceased.

We note that because the situation is not before us, nothing in this opinion should be taken to permit the states to bind electors to a deceased candidate, the court said.

Delaying the election would be one way to ensure the process could be restarted with the different candidates, but no presidential poll in us history has ever been pushed back.

In july, mr trump raised the possibility. in the context of his unsubstantiated warnings about fraud, he wrote on twitter: delay the election until people can properly, securely and safely vote???

However, the power to delay rests in the hands of congress, not the white house.

Lawmakers in the house of representatives and the senate could vote to alter the date of the presidential election by changing the relevant statutes, according to the national constitution center.

But they could not delay the vote indefinitely. the constitution does not specify what day the election has to be held but it does include a hard deadline: january 20. that is the day when the presidents term in office has to end.

The presidential line of succession is much clearer than the question of replacing a presidential candidate mid-campaign. first in line to replace mr trump is mike pence, his vice-president. after him comes nancy pelosi, the democratic speaker of the house of representatives and one of mr trumps chief opponents. next in line is republican senator chuck grassley of iowa, the president pro tempore of the senate.

If the presidents ability to perform the duties of office was impaired because, for example, he was to be put on a ventilator, the constitution allows him to notify congress that the vice-president will become acting president.

According to john hudak of the brookings institution, the relevant constitutional provision section 3 of the 25thamendment was invoked once by ronald reagan and twice by george w bush for medical procedures in which anaesthesia or heavy sedation was used.