Barack Obama swears on Abraham Lincoln’s Bible, held by Michelle Obama, while he is is sworn in as the 44th president of the United States on Jan. 20, 2009. (AP)

As I was wrapping up a phone call with my mother recently, she mentioned she had just one more question for me: Exactly what happens if a U.S. presidential candidate decides to withdraw from the race after the nominating convention has passed?

After about 15 minutes of trying to go through the various scenarios, I realized that I needed some expert help. Fortunately, my colleague Richard Pildes, a New York University School of Law professor who is one of the nation’s leading authorities on election law and author of “Romanticizing Democracy, Political Fragmentation, and the Decline of American Government” was kind enough to answer my questions via email. What follows is a lightly edited version of our discussion.

Let’s work backward in time. What happens if the winner of the election either dies or withdraws after Congress has counted the votes from the electoral college in January, but before the president-elect assumes office?

The first part is easy, if a president-elect dies before assuming office. The 20th Amendment resolves this problem directly: The vice president-elect becomes president.

The 20th Amendment only specifically addresses the death of the president-elect, not the voluntary withdrawal of a president-elect. Presumably, a president-elect who wanted to withdraw could wait until Jan. 20 and do it then. At that point, Article 2 and the 25th Amendment make it clear the vice president would become president.

So what happens if this takes place instead after the electoral college has voted in December, but before Congress formally has received and counted those votes?

Now things start getting more unclear. If the person who wins the electoral-college vote is considered “the president-elect” even before Congress officially receives and counts the vote, then the answer is the same as in the previous circumstance.

But legally, it would not be clear whether someone is the president-elect before Congress formally receives and counts the votes of the electors in January. If the winner of the November election and in the December electoral-college tally is not considered the president-elect and dies before January, then no clear legal answer is provided by the Constitution or by statute.

I would think there would be tremendous popular pressure to treat the electoral-college winner as the president-elect — and that courts would not stand in the way of the system resolving the situation here the same way as before, with the vice president-elect becoming president.

So how about if the winner of the November election dies or withdraws before the electoral college meets in December?

Now things become even messier.

The issue is how an elector should or can cast their vote. Should the elector vote for the dead winner of the election, if the elector otherwise would be obligated to do so? Should he or she vote for the vice-presidential candidate of that party instead? How would these votes be tallied in Congress?

In addition, there are questions about what various state laws would permit. Do they permit, for example, an electoral-college representative to vote for the vice-presidential winner in this circumstance?

And beyond that, there are serious questions about whether it is even constitutional for state laws to purport to bind their state electors to vote in a certain way, which would also come into play at this point.

So that leaves the scenario people are talking about at the moment. What if a party’s nominee dies or voluntarily withdraws before the November election?

Three layers have to be unraveled on this question. First, and probably most important, is how a party goes about replacing its nominee. In the Democratic Party, the formal decision-making body is clear. The chair of the Democratic National Committee (currently Donna Brazile) would call a special meeting of the DNC, which is roughly a 447-person body. That body has the power to replace the party nominee, as far as the party is concerned. This is how the Democratic Party replaced Thomas Eagleton with Sargent Shriver as the VP candidate after the 1972 convention.

But how the DNC goes about making the choice — under what rules, through what process — is not spelled out further in the party rules.

On the Republican side, it would also be the Republican National Committee that would have the power to choose a replacement nominee (though the RNC is a smaller body, around 150 members, than the DNC).

Second, once the party comes up with a new nominee, the question becomes whether that candidate can now get on the ballot in various states to replace the convention’s nominee. This is an issue of state law, handled differently in different states. In some states, it is formally too late at this point to replace a party nominee for the presidential election. But the courts might well conclude that state laws that allow too short a time for replacement in the case of death or withdrawal, in a presidential election, are themselves unconstitutional.

Third, and finally, we are back to the question of how the electors vote. Suppose the convention’s nominee cannot be replaced on the ballot in time but has died or withdrawn. The party has chosen an alternative, through the process above, but that person can’t get on the ballot. And now voters who support the party — let’s say Democrats — vote for the Democratic candidate on the ballot, even though he or she has withdrawn, to express their support for the Democratic Party. What does the elector do?

We are now more or less back to the question and the scenario I described in the third case. Presumably the elector would like to vote for the person the Democratic Party has chosen to replace its nominee and does so. If state law prohibits that, is the state law unconstitutional? Is Congress obligated to accept this vote?

Any final thoughts or recommendations for where people can go to learn more about this topic?

There should not be this much uncertainty about such momentous questions, even if the events involved are low-probability events. The political parties need to clarify their rules after this election. Congress should consider legislation to resolve some of the remaining uncertainties.

For more information, see the article “Presidents, Vice Presidents, and Death: Closing the Constitution’s Succession Gap” by Akhil Reed Amar.