GEOFF BENNETT: The U.S.

Supreme Court heard arguments today in a case that could reshape how millions of mail-in ballots are counted in this fall's elections.

Our justice correspondent, Ali Rogin, has more on the ensuing legal background.

ALI ROGIN: Geoff, currently, 14 states and the District of Columbia allow elections officials to count mail-in ballots that arrive after Election Day, as long as they are postmarked by Election Day.

But the Republican National Committee is challenging that procedure, arguing it undermines trust in elections.

In today's arguments, the justices appeared to be divided along ideological lines.

SAMUEL ALITO, U.S.

Supreme Court Associate Justice: Labor Day Memorial Day, George Washington's birthday, Independence Day, birthday, and Election Day, and they're all particular days.

So if we start with that, if I have nothing more to look at than the phrase Election Day, I think this is the day in which everything is going to take place.

KETANJI BROWN JACKSON, U.S.

Supreme Court Associate Justice: This idea of the votes being cast by Election Day and counted after Election Day has been around, right?

I mean, it's not like we're talking about a brand-new thing from Mississippi from the standpoint of no one ever had a post-Election Day ballot deadline before.

ALI ROGIN: If the court sides with the RNC, it could significantly change voting rules just months ahead of this year's midterms.

For more on the fight over voting by mail, I'm joined now by Nate Persily, an election law scholar at Stanford University.

Nate, thank you so much for being here.

What is the core argument here?

Why are Republicans fighting against this law?

NATE PERSILY, Stanford Law School: Well, what the Republicans are saying is that there is a federal law that sets the date for the election as a particular day, and so they are suggesting that any ballots that are received after that day should not be counted because the federal law trumps the state law, in this case, Mississippi's law.

ALI ROGIN: And how did this become such a partisan idea?

Is there a partisan preference for mail-in voting in one party over the other?

NATE PERSILY: In the last decade, we have seen a partisan polarization on the issue of mail balloting, in no small part because Donald Trump has made the central part of sort of his election reform agenda and claims of massive fraud that he suggests that mail voting is inherently insecure.

And so you have seen Republicans in general being more restrictive on issues of mail voting and Democrats being more liberal.

I should say that's not uniformly true.

Places like Utah, which is a heavily Republican state, have all mail voting.

And so both Republicans and Democrats take advantage of that as a way of casting their ballots.

ALI ROGIN: If the court rules against this state law, what happens in terms of elections come the fall?

Does anything change immediately?

NATE PERSILY: Well, we will see how they phrase the opinion.

One would expect that this decision would come down around June.

That would definitely throw a wrench in the gears for a lot of the election planning at the states.

But what it would mean is that states would not be able to count ballots after -- if they're received after Election Day.

And so they would have to change some of their state laws.

And you may actually have a situation where in some states they will count ballots for state offices like governor after Election Day, but they won't be able to count congressional races after Election Day.

ALI ROGIN: Right.

That was a big part of the argument too, whether it's a state election or a federal election.

We heard a lot today about what defines Election Day and that conversation included a lot of talk about things like early voting.

So is there a chance that this court ruling could extend beyond mail-in ballots and have farther reaching implications for how we run elections?

NATE PERSILY: Well, one of the problems with the Republicans arguments here is that, if you really believe that the casting and counting of ballots happens has to happen on one day, then that would seem to not only affect the receipt of ballots after that day, but also the casting of ballots beforehand.

However, they said, no, they wouldn't take that argument that far.

And so everybody in the case agreed that early voting, which is to say casting of ballots and giving them into the election officials before Election Day, would still be allowed.

But whether -- the sort of liberals on the court were trying to say is that, look, if you really believe Election Day means Election Day, that should apply to the counting of ballots and the casting of ballots before Election Day as well.

ALI ROGIN: How does this case play into the Supreme Court's broader efforts to reshape election law?

And to what extent is the Supreme Court here taking over making policy where Congress also has a role to play?

NATE PERSILY: Well, that's a very good point to make, which is that this is a statutory case.

The only question in this case is whether the federal law passed by Congress is inconsistent with the state law passed by Mississippi that would allow for ballots to be received and counted after Election Day.

And so everyone agrees that Congress could have clarified this to make clear that you couldn't have ballots received after Election Day.

The question is whether a vague law that just declares the Election Day as being a particular Tuesday in November, whether that says something about the ability to count ballots after the fact.

ALI ROGIN: And, of course, at the same time, Congress is talking about the SAVE Act, which is a big priority for President Trump and would significantly affect the way that Americans can vote.

What is the context here in terms of one of these battles playing out in the Supreme Court and the other making its way through Congress?

NATE PERSILY: Well, so the SAVE Act does have provisions related to mail balloting, but it goes much farther.

It deals with things like voter I.D.

in the polling place, as well as citizenship requirements for registering.

And so one of the questions here is whether Congress would be able to essentially go farther than even those in this case are suggesting to have a sort of greater federal law that would supplant a lot of these state laws with respect to voter access and the casting and counting of ballots.

ALI ROGIN: Nate Persily with Stanford University, thank you so much for breaking this down for us.

NATE PERSILY: Thanks for having me.