Many states have shifted their elections to include more voting by mail in an effort to keep people from congregating at polls, but competing claims and different definitions of the systems these states are adopting have led to warnings of fraud by Republicans and accusations of voter suppression by Democrats in response.

At the heart of the debate is how to define "universal mail-in," "absentee" and "no-excuse absentee" voting; whether there are differences among the three terms; and if those systems present risks of voter fraud or disenfranchisement.

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Democrats have argued there is little difference between universal mail-in voting and absentee voting – as they both involve the same fundamental process of casting a ballot using the mail. Democrats also cite numerous studies showing no evidence of significant levels of fraud in any form of mail-in voting as Americans have largely embraced the method in recent years, casting almost 25 percent of their votes in 2016 using the mail, according to the Brookings Institution.

But Republicans have argued that with more than half of Americans expected to vote by mail in 2020, state systems could be overwhelmed. They also say there is a distinction between the systems and that universal mail-in voting opens up opportunities for bad actors to commit fraud.

Here's what to know about the differences between absentee voting and universal vote-by-mail.

Absentee

The most traditional definition of absentee voting involves a voter who is for some reason unable to make it to the polls on Election Day. The voter obtains a form to request a ballot, fills that form out with their excuse for not being able to vote in person and sends it to the state. The state sends the voter a ballot and the voter finally returns the ballot with his or her vote.

Absentee ballot request forms for states with this kind of system are available at county election boards or can sometimes be downloaded online. Most of the time, the acceptable excuses for standard absentee voting are being out of the state and having medical conditions.

"No-excuse" absentee

"No-excuse" absentee voting allows voters to request a mail-in ballot without providing a reason why they need one, making the voting method more accessible to people who may simply prefer to vote that way.

"There's no reason for it other than that is your personal preference," Darrell West, the vice president and director of Governance Studies at the Brookings Institution, said of no-excuse absentee voting.

Darrell West of the Brookings Institution warned that the most likely problem associated with increased mail-in voting could be states struggling to verify signatures rather than fraud. (Brookings)

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Some states, like Maryland and Michigan, add a wrinkle to their no-excuse absentee systems by sending all voters the absentee ballot request form. This takes out the step for the voter of obtaining the ballot request in the first place, thus making it easier to vote absentee.

But no-excuse absentee voting still requires affirmative steps on behalf of voters to tell the state that they want a mail-in ballot before the state actually provides them with one.

Universal vote-by-mail

Universal vote-by-mail removes all the hurdles associated with the different forms of absentee voting, sending the actual mail-in ballot to every registered voter in the state. Voters don't need to do anything to get that.

West called this the "most contentious category" of voting, noting that President Trump "worries there's going to be shenanigans associated with it."

To be clear, there are in-person voting options in these universal vote-by-mail states. But registered voters will still get mail-in ballots sent to them no matter how they choose to vote. And by shifting to universal vote-by-mail, states are making clear they want voters to take advantage of the system, especially in those states that are making the change explicitly because of the coronavirus pandemic.

There were some states that previously used universal vote-by-mail without much issue, including Colorado since 2013 and Oregon since 2000.

"There are a number of places that have lengthy experience with voting by mail, and they've had no problems and there's been almost no fraud associated with that," West said.

But other states, including Nevada and New Jersey – Nevada via legislation and New Jersey via an order by Gov. Phil Murphy – have adopted the system this year in direct response to the pandemic. That concerns West.

"I think there is some question about the states that are doing it for the first time, whether they're going to have procedures in place," West said. "I think one of the big issues this year is going to be the signature verification... I think this is the thing people should be focused on, because that's where the litigation is going to occur and where the controversy is going to be."

Competing claims, disenfranchisement concerns

After some initially confused messaging, Republicans have made clear that they back absentee voting in its various forms. Trump tweeted in July that "Absentee Ballots are fine because you have to go through a precise process to get your voting privilege."

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But Trump added: "Not so with Mail-Ins. Rigged Election!!! 20% fraudulent ballots?"

There is no evidence to support Trump's claim that 20% of votes in a universal mail-in election could be fraudulent. "I'm not very worried about that just because there are a lot of safeguards built into that part of the process. And of course, anyone who violates that is subject to election fraud indictment, and that can carry either a hefty fine or prison time," West said. "So I'd be surprised if that were the issue."

Nevertheless, Trump and his campaign have stoked fears over ballot harvesting, which is a practice in which politically motivated individuals or groups collect large amounts of mail-in ballots, often from vulnerable individuals like the elderly, and turn them in en masse.

"Because of these widespread inaccuracies in a state’s voter registration records, a state that sends ballots to all registered voters will necessarily send ballots to persons ineligible to vote or others with fake registrations, invalid registrations, outdated registrations, and to the deceased," the Trump campaign said in a lawsuit against New Jersey, one of several suits it's filed against states planning on universal mail-in voting. "These risks are compounded by the practice of ballot harvesting – that is, coordinated efforts to have third parties collect absentee ballots from voters and drop them off at polling places or elections centers."

Democrats have disputed that universal vote-by-mail could make it easier for ballot harvesters compared with absentee voting, and even that there is a difference between universal vote-by-mail and absentee voting.

"[L]et’s be clear, there is absolutely zero difference between voting by mail and voting absentee," Michigan Secretary of State Jocelyn Benson said at the Democratic National Convention. "Millions of Americans have been voting absentee for decades. Donald Trump, his family, his staff, they all vote by mail. In fact, in states like Colorado, Utah and Oregon, voters have been voting by mail for years. Republicans and Democrats agree, it is safe."

Benson, whose state is a no-excuse absentee state, appeared alongside California Secretary of State Alex Padilla. California is a universal vote-by-mail state, which is a different system.

California Secretary of State Alex Padilla and Michigan Secretary of State Jocelyn Benson spoke at the Democratic National Convention (Official)

"Grab your mask and head to the polls the first day they’re open or request your ballot and send it in right away," Padilla said. "And know this, election results may take a little longer this year, but Democrats will fight to make sure your ballot is counted."

But there have been examples of such fraud disrupting elections on a small scale just this year, including in Paterson, N.J., where four people were charged with fraud over ballot harvesting efforts, eventually causing a redo of a city council election. It's unlikely such efforts could potentially be widespread enough or scaled-up enough to significantly affect a presidential election, but any fraud is considered concerning.

There have also been concerns about whether the Postal Service can handle the increased amount of mail-in ballots expected this year. Still, even if every voter in the presidential election voted by mail – a process that would be spread out over weeks – it would add up to just a fraction of the 472.1 million pieces of mail the U.S. Postal Service says it delivers per day.

"I want to make a few things clear: The Postal Service is ready today to handle whatever volume of election mail it receives this fall," Postmaster General Louis DeJoy said at a Senate hearing this month. "Even with the challenges of keeping our employees and customers safe and healthy as they operate amid a pandemic, we will deliver the nation’s election mail on time and within our well-established service standards."

Perhaps the biggest concern with states ramping up to handle almost all of their ballots by mail, however, is accidental disenfranchisement rather than malicious fraud.

A CBS investigation from July mailed mock ballots to simulate a vote-by-mail election on a small scale, and 3% of the ballots never made it back, meaning those voters would not have had their votes counted. And on a larger scale, the Associated Press reported that more than 100,000 mail-in ballots were rejected by California election officials during the March presidential primary.

Tens of thousands of the ballots mailed either didn't have a signature or the signature on the ballot didn't match the voter's signature. But the most common problem is that voters missed the deadline for the ballot to be mailed and arrive. To count in the election, ballots must be postmarked on or before Election Day and received within three days afterward.

"I think the problem is going to be if people wait until the last minute to send them their mail ballots, there could be delivery problems associated with that," West told Fox News. His concern echoes letters sent by the Postal Service to most U.S. states warning that their mail-in ballot deadlines could lead to ballots not being counted because they could not possibly be delivered in time even if voters follow the rules. The Washington Post first reported the story.

“The Postal Service is asking election officials and voters to realistically consider how the mail works,” Martha Johnson, a spokeswoman for the USPS, said in a statement.

Then there's the issue of validating mail-in ballots that are going to be sent at a much higher volume in this election than in previous years.

"Local officials have a fair amount of discretion in validating or invalidating signatures," West said. "And of course, that's a tricky business... We see that in trials where there are expert witnesses whose job is to identify particular signatures."

West added: "There often are disagreements even among experts in the election process. There's gonna be an issue in that the signatures that are on file might be quite old and people's signatures change over time... It's going to create a lot of havoc just determining what is a valid signature."

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But the progressive Brennan Center for Justice has argued that vote-by-mail is still the best solution for an election being held during a pandemic.

"The coronavirus has made congregating in small, enclosed spaces dangerous. At many polling places, voters – particularly of color and from poorer communities –already wait in long, crowded lines to vote," Matthew Harwood wrote in a post earlier this year arguing for at least a vote-by-mail option. "During a pandemic, such lines would force citizens to choose between their health and their right to vote."

Harwood added: "There is ample experience to show that a vote-by-mail option is safe and gives citizens the ability to participate."

Fox News' Gregg Re, Morgan Phillips and the Associated Press contributed to this report.