Analysis: Here's what to know as mail-in voting begins

That shift is playing out against the backdrop of consistent and unfounded attacks from President Donald Trump, who this week encouraged people in North Carolina to test the system by voting twice -- once by mail and once in person, which election officials said would be illegal.

But the shift away from in-person voting will accelerate the election timeline. Almost every US state has a mail-in ballot application deadline in October:

So what does this mean for Election Day? As CNN's Joan Biskupic reports: Some Democratic lawyers have begun arguing that states could take until late December to submit their Electoral College totals. Some Republican lawyers counter that earlier dates are hard and fast. Election law experts, meanwhile, note that Congress could change the archaic law at any point and establish new deadlines.

The bottom line: The one date fixed by the US Constitution is January 20, at noon, when the four-year term of the president and vice president ends and the next begins.

Preparing for a long election

CNN spoke with Rick Hasen, a University of California, Irvine, professor and new CNN election law analyst, about the unusual 2020 election process playing out in real time. The conversation, conducted over the phone and lightly edited for flow, is below.

CNN: Do you think as a country, we are starting this process early enough, given the expected flood of absentee ballots?

RH: I think the process is starting early enough. That's not my main concern in terms of people being able to vote by absentee ballot. The main concern is that people don't request them soon enough in places where they need to make a request, because what we've seen in the primaries is that many voters waited until later in the process when they're more likely to be paying attention to the election and that a combination of inundated election officials, slow Postal Service delivery and procrastination by voters left us with thousands of voters who failed to get their ballots returned on time and who were then effectively disenfranchised.

Because somebody who gets their ballot in early September will easily be able to return it in time, either through the US mail or, if allowed under state law, to an official site or drop box. It's the people who wait where there's the problem.

CNN: Trump has repeatedly cited New York as an example of problems with mail-in voting. How applicable do you think it is to look at some local elections as indicators of how things might run on a national level?

RH: Well, New York was among the worst states in terms of being able to handle a flood of mailed-in ballots.

To some extent, the more experience a state has with mail-in balloting, the quicker they get it done. States that are not familiar with the process sometimes had problems. That said, many states were able to handle a large influx of mailed-in ballots and report results relatively quickly.

I think people don't understand that the reason for the delay in reporting of absentee ballot results is that there are security measures in place to make sure that the ballots are valid and were sent in by the voters who were supposed to be sending them in. And so it takes time to process them. And that's why we might not have results -- definitive results -- of the election on election night. But some states obviously do a better job than others, and New York was one of the worst-performing states in terms of competence in processing, accounting, absentee ballots.

CNN: Some people might associate a delay in counting ballots with something going wrong or being problematic, but you're saying it's actually a result of the election integrity process.

RH: Exactly right. In fact, I was part of an ad hoc committee that issued a report in April called Fair Elections During a Crisis. And in the report, we said that a delay in the full processing of the absentee ballots is to be expected when you're dealing with states that are going to face many of these ballots and don't have long experience counting them. The delay does not mean that something nefarious is going on, but instead is the sign of a process by which election officials are being careful and making sure they're only accepting validly completed ballots.

That said, election officials need to be transparent about their process of handling absentee ballots and should be giving regular updates to the public as to how many ballots have been counted, what remains to be counted and from where.

CNN: When we last spoke in May, you said that media companies and people with platforms need to prepare people for the possibility that we won't know who won the election on Election Day. Do you think at this point a critical mass of people understand that?

RH: Well, just to clarify what I said: We may know the winner of the election. It depends on the margin. So the reason we would not know the winner on election night is if it is close enough in a state or states that are crucial to the Electoral College outcome, where there are many outstanding absentee ballots to be counted.

So that's not necessarily the situation we'll face in November. It could be, for example, that Pennsylvania is extremely close and we'll take a week or two to figure out who has won that state's Electoral College votes.

But if Trump or Joe Biden could win without taking Pennsylvania into account, then I think we could well see an election where the media has appropriately called that race even if we don't know the full Electoral College count.

And so do I think that this message has been getting through? I certainly think it's been getting through to the media. There's been a lot of reporting by journalists about this issue. And I think the media is now much more educated about the fact that with new states facing a potential flood of mail-in ballots, that there could be a delay in the results.

In terms of the public, I did see that an Axios Ipsos poll found that 36% of Americans think we'll know who won on election night; 60% expect the winner to be announced within a couple of days. I mean, the fact that there are significant percentages of people who don't expect that I think is a good sign. It means the message has gotten through that the race could be too early to call if it's close.

CNN: Do you have concerns about the long-term effects of the unfounded claims about voter fraud coming from Trump as we head into the final months until Election Day?

RH: Well, first and foremost, I'm worried about his supporters accepting the legitimacy of the election in the event that he loses. I'm also worried about the statements he's making causing Biden supporters to be less willing to accept the results of the election if Biden loses.

So first and foremost, I'm worried about the integrity and the accepted legitimacy of the 2020 election. But I do think that these have far-reaching implications, because if you have a significant percentage of the population that believes that our elections are routinely rigged or stolen, then it's hard to see how you can have a functioning democracy.

Democracy depends upon the losers believing that the election was run in a fair way and agreeing to fight another day. And yet that is not something that we should take for granted any longer, given the kinds of unsupported statements of widespread fraud that the President has made.

He often will point to relatively small instances of fraud or point to instances of election problems that don't involve any proof of fraud and generalize statements about the lack of integrity of the entire process. I think that's dangerous for democracy, when you have a political leader consistently casting doubts on the legitimacy of the electoral process without evidence to back it up.

CNN: Trump already said last month, "The only way we're going to lose this election is if this election is rigged."

RH: That's right. He did make that statement. But remember, this is the same President who said, without any evidence whatsoever, that 3 to 5 million noncitizens voted in the 2016 election, all for his opponent, Hillary Clinton.

CNN: Which was an election he won.

RH: Right. So he cast doubt on the legitimacy of an election he won. Imagine what he might do for an election he has lost.